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Reverse Polarity
aka: Reverse The Polarity

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"Lewis did it! Oh, thank the gods for legitimate, fact-based science!"

Harley Quinn: [holding a raygun] Don't come any closer, or I'll... [reads label on her raygun] reverse polarity!
[odd look]
Harley Quinn: I don't know... but it always works on Star Quest.
Gotham Girls, "Gotham in Blue"

When a major obstacle in a Science Fiction show is resolved purely through the judicious application of Techno Babble, the characters have successfully Reversed the Polarity. It seems that every futuristic gadget or space ship subsystem performs some miraculous function if only you route the power through it backwards. Urban legend has it the expression originated on Doctor Who. In reality, the phrase "Reverse the Polarity" can be traced back at least as far back as the 1898 The War of the Worlds sequel Edison's Conquest of Mars by Garrett P. Serviss. However, it was popularized by Doctor Who, as Jon Pertwee asked the writers for a simple piece of Technobabble he could reliably deliver. The version most associated with the Third Doctor is "reverse the polarity of the neutron flow", although he only said it once during his time as the Doctor and once in a multi-Doctor special.

A type of Applied Phlebotinum, Reversing the Polarity is the be-all end-all technical solution for any problem. Usually only thought of at the very end of the show ("Captain... we could reverse the polarity of the positron toilet and send a stream of charged crap particles toward the Romulans, rather than away..."). It always works. Always.

Of course, you can "reverse the polarity" in real life—just put the battery in the other way round. Doesn't quite have the same effect, though. Most simple powered toy vehicles, electric toothbrushes and other devices that rely on a spinning electric motor will simply run backwards while more complex electronics with a DC power supply may even break or fry the device in question. This is why most "complex" devices nowadays are equipped with diodes, which keep the current from flowing backwards if the polarity is reversed, preventing damage to the main circuitry. Still, Don't Try This at Home, kids! Don't even bother trying it with Mains power either: Alternating current means the polarity is already being reversed, 100 or 120 times a second depending on country. You'll notice that it doesn't make your hairdryer or vacuum cleaner go backwards.

Reversing the polarity on a car is also possible—some vintage cars, particularly British ones, are positive-ground, while negative-ground has been the standard worldwide since The '60s, so if you want to put a modern MP3-compatible stereo in your '59 Morris Minor a car polarity swap is a must.

Besides, it makes the characters in question look a lot less like brilliant scientists when they are (basically) sighing and asking the technician, "Did you plug it in the wrong way again? I mean, seriously. Red cable is positive. How hard can it possibly be to remember?"

Of course you can't do this with neutrons, because they're as electrically neutral as the name suggests, although if we're getting technical, they do have a magnetic polarity. However, if the neutrons are flowing somewhere, reversing the polarity might refer to changing the direction of flow.

Closely related to the Forgotten Superweapon. Also see Technobabble and Revive Kills Zombie.

Compare to Tim Taylor Technology and Had the Silly Thing in Reverse.


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    Anime and Manga 
  • Neon Genesis Evangelion: In episode 13, a continuously evolving Grey Goo gets into NERV headquarters by hitching a ride on one of the EVAs. They then form themselves into an organic supercomputer, which starts to hack into the Magi supercomputers that run the headquarters. To prevent them from resulting in the self-destruction of the Magi, Ritsuko has to open them up, climb inside one of them, and reprogram it from the inside.
  • This once happens in Pokémon: The Series's English dub when Team Rocket tries to get hold of two Luvdisc, and then reverse the polarity to get rid of all of the love in the world. This being Team Rocket, it's pretty much an Affectionate Parody of the trope. The original version just has them wanting the Luvdisc captured for an ongoing Team Rocket project.
    • In Pokémon: Jirachi: Wish Maker, Butler makes a machine that is supposed to create a live Groudon from its fossilized remains. When the machine creates an enormous evil monster instead, he is able to make the machine destroy Groudon by simply reversing the direction of the fossil and the levers.
  • Uchuu Senkan Yamato: This was how they got the drill-missile to unscrew itself out of the barrel of the Wave-Motion Gun.
  • In Transformers Victory, Braver invents a device that can detect Decepticon brainwaves. During the inevitable battle, he is able to drive them off by actually reversing the polarity.
  • In the Cowboy Bebop episode "Honky Tonk Woman":
    "She reversed the polarity of one of the missiles! It's coming right back at us!"
  • In episode 17 of the Sakura Wars (2000), Kohran puzzles over how to make Iris's kohbu properly handle her vast spirit energies, and comes up with an idea that, among other things, reverses the flow of her spirit energy through the regulator crystal.

    Comic Books 
  • Subverted in Runaways: Victor Mancha (who is being held captive by the aforementioned group) attempts to escape by threatening Gertrude York with a remote that has had its "polarity reversed". The other Runaways scoff at the idea.
    Gertrude: Relax, people. He's a powerless kid holding a remote control.
    Victor: I... I flipped this thing's vibranium battery when you weren't looking. If I press one button while the polarity is reversed, it... it won't be pretty.
    Molly: Yeah, right. Even I know he's tricking us, and I dropped out of the fourth grade.
  • Superman
    • In War World, Superman destroys the eponymous weapon-satellite by reprogramming its computers and redirecting its defense-systems in on itself.
      Superman: Despite all our much-vaunted power, Kara and I wouldn't stand a chance of defeating Warworld in a head-on confrontation — so I've done the only other thing possible: I redirected the satellite's defense-systems in on itself!
    • In Supergirl: Cosmic Adventures in the 8th Grade, Lena stops a core reactor from reaching critical mass and exploding by reversing the polarity:
      Lena: Atomic batteries to power... Reversing polarity of the neutron flow... activating quantum dimensional recall signal... Now!
    • The Death of Luthor: Supergirl finds an alien world whose atmosphere has been crystallized by the beams of a hostile spaceship. After trashing the robots who were manning the ship, Kara restores the atmosphere to normal by studying their equipment and flipping a switch which causes the crystallized air to dissolve.
    • The Earthwar Saga: Since Chameleon Boy has been frozen into his real form by one space pirate's ray gun, Superboy grabs the weapon, reverses the controls and shoots his teammate, successfully breaking his shapeshifter mode lock.
  • Disney Ducks Comic Universe: A Carl Barks comic is the earliest recorded use of Reverse Polarity as technobabble, dating all the way back to March 1961 (US #33) in "Billions in the Hole".
    Engineer: "I have to de-amp the resistor diodes to reverse the polarity of the potentiometers — it's simple when you know how!"
    Dewey: We'll take your word for it, sir.
  • Atomic Robo's solution for sending the Vampires back to the Vampire Dimension is to reverse polarity on the experiment that accidentally brought them into our world. Unfortunately, he finds that the machine does not have a "reverse" option. Robo considers this almost criminally negligent with the sort of ludicrous science they get up to at Tesladyne, and declares that the feature is to be strict company policy from then on. Lampshade, thou art hung.
  • Buck Rogers. Did it way more than Star Trek ever did, too.
  • In Sonic the Comic Tails states lines involving this a few times, which is unusual since he isn't a child genius in Fleetway's continuity.
  • In the She-Hulk graphic novel, when it becomes obvious that the damage done to the S.H.I.E.L.D. Helicarrier can't be repaired in time to prevent it from crashing, a scientist tries to cushion it by reversing polarity on the underside tractor beams, turning them into repulsor beams which push against the ground rather than pull from it. (Even so, it's not a very soft landing, but at least he prevents the core from rupturing.)
  • In issue 52 of Star Wars (Marvel 1977), the heroes barely escape from the Empire's superweapon of the week, an ersatz Death Star called The Tarkin. When it tries to fire on the Millennium Falcon, it suddenly explodes. Leia reveals that she switched at couple of wires to reverse the polarity modes of the cannon's fire controls.
  • In Michael Moorcock's Multiverse, the Chaos Engineers manage to reverse the polarity of the multiverse itself, effectively reversing everybody's destiny.

    Fan Works 
  • Early in Rhyme and Reason, in one of her trademark Techno Babble fits, Gadget talks about tracking burglars with sound using reverse-polarity speakers.

    Film — Animated 
  • In Cars 2, Holley says this while trying to escape a death trap in the Big Bentley clock tower. On one hand, they at least keep it grounded in reality: once reversed, the only effect is the clock's motor and gear system running in reverse. On the other hand, she does it by shocking the motor with a Taser...
  • In The Transformers: The Movie, the heroes' ship is targeted by Decepticon missiles. Kup's solution is to reverse the polarity, like he did against the Shrikebats of Dromedan. Hot Rod is afraid it will tear the ship apart; instead, it repels the missiles.

    Film — Live-Action 
  • Used to defeat energy-eating giant square stompy robot in Kronos (1957).
  • In Austin Powers in Goldmember, the tractor beam used by Goldmember and Dr. Evil to pull an asteroid to Earth has "Do not reverse polarity" written on a low-tech-looking panel. When Dr. Evil, having turned good, does reverse the polarity, the beam destroys the asteroid instead of sending it to Earth.
  • Ocean's Thirteen has Rusty dismissing Basher's technobabble by saying, "Becomes magnetized, reverse polarization, I know."
  • Batman: The Movie: "If I could just... reverse the polarity... send out waves... of super-energy!" Highly effective on torpedoes until... "Confound it! The battery is dead!"
  • In the 1952 British film The Sound Barrier, the hero solves the problem of controlling a supersonic airplane by reversing the flight controls. To increase lift, for example, he pushes the stick forward instead of back as would be expected. This is based on the real-life phenomenon of "control reversal", which is detailed in the Real Life section below.
  • In The Avengers (2012), the phrase "reverse the polarity" is used by Tony Stark in a less unacceptable context — Stark is giving Captain America instructions to slow down a turbine engine/motor. As discussed above, reversing the polarity on a basic DC motor will reverse the direction of its rotation, which has the immediate effect of slowing its motion.
  • Spy Kids:
    • In the first Spy Kids movie, the heroes force a Heel–Face Turn of the titular Spy Kids robots by reversing their alignment polarity. He does this by inverting the binary code. While it probably would stop the robots from attacking, they would be more likely to crash then turn good. Of course, you can Never Say "Die" in a kids film, so...
    • In Spy Kids 2: Island of Lost Dreams, Gerti disables the amusement park ride in the opening by reversing the polarity, justifying it by saying that the ride works using electromagnets.
  • In Young Frankenstein, Frederick reads from his Doctor Frankenstein grandfather's book How I Did It:
    "'...until, from the midst of this darkness, a sudden light broke in upon me. A light so brilliant and wondrous and yet so simple. Change the poles from plus to minus and from minus to plus. I alone succeeded in discovering the secret of bestowing life. Nay, even more... I myself became capable of bestowing animation upon lifeless matter!' ...IT - COULD - WORK!!!"
    • Some fans justify it in that, being at the end of the journal, we don't hear all of it. It can be assumed that the previous Doctor Frankenstein's experiment failed and that he eventually figured out the solution, which was revealed in the excerpt. In other words, he had something plugged in backwards.
  • In Frankenstein Meets the Wolf Man, Dr. Frankenstein's journal explains that the practically immortal monster can be killed by attaching him to the machine that gave him life and "changing the poles".
  • Son of Flubber features a rain machine that does not work until they reverse the polarity.
  • In the 1996 TV movie Gulliver's Travels, Gulliver reverses the big magnet in the flying island to counter the plot of the people on the ground to crash the island by using another big magnet that attracts the island.
  • Self-inflicted in Casino Royale (1967) when a henchman with a crude battery-powered pacemaker is unplugged by Joanna Pettet's character. He frantically reconnects himself and gets the leads wrong, running backward at high speed.
  • In Superman II, Superman reverses the chamber that takes away Kryptonian powers. Instead of taking away the powers of the person inside the booth, it removes the powers from the three Kryptonians (Zod, Ursa and Non) standing around outside with Lois and Lex Luthor while he was safe inside. Lex Luthor even comments that "He switched it...".
  • Star Trek: Generations: Data literally uses the phrase "reverse the polarity" when he opens a magnetic door by attenuating his axial servo. Of course, reversing the polarity on a magnetically controlled door would open it. Go figure.
  • Ghostbusters:
    • In Ghostbusters (1984), the Ghostbusters defeat Gozer by crossing the energy streams from their proton packs. This, we are informed, will "reverse the particle flow through the gate." Naturally it works. This is an echo of Egon's earlier warning that crossing the streams would cause "total protonic reversal", which presumably means protons flipping to a negative charge (perhaps indicating that they're creating antimatter).
    • Ghostbusters (2016) has a similar case that is better explained — there is a portal releasing paranormal energy, so the Ghosbusters decide to do a "total protonic reversal" on that to make the breach work the opposite way, sucking the ghosts in like one of their traps.
  • Forbidden Planet: Commander Adams orders a subordinate to "Stand by to reverse polarity" during the initial landing on Altair 4, apparently referring to flipping the engines from "push forward through space" to "push back against gravity". Until an earlier example is found, this 1956 film may be the first time the trope was used.
  • One of the many reasons why Fantastic Four (2005) was criticized was its wham-tastic use of this trope. Reed is constantly spouting technobabble, and quite literally, his plan to return the Four to normal is to reverse the polarity of the cosmic rays that gave them their powers.
  • In Endless Descent/The Rift, Wick instructs Robbins to "reverse the polarity of the ship's radar cloaking device" when it's being attacked by an undersea creature.
  • Combined with Apocalypse How in R.I.P.D.. The Staff of Jericho makes the gate to the afterlife (a giant fan) spin in the opposite direction and send all the dead back to Earth.
  • In Time Chasers, the time machine normally goes forward in time; reversing the polarity causes it to go backward in time.
  • In Apollo 13, the crew reverses the polarity of the power link between the Command Module and the Lunar Module in order to get enough power to restart the Command Module systems for reentry. Unlike most examples, this was a real-life necessity, as the LM's power link was intended to power the LM from the Service Module until separation, and reversing the polarity allowed the command module to draw power from the LM.
  • Lucas in Dinosaur Island (2014) claims, amongst other Technobabble, that reversing the polarity of the magic crystal the heroes found is one of the things that will help them escape the island.
  • Resident Evil: The Final Chapter: The group enter Hive via an air intake, which requires easing themselves between the blades of a giant turbine. Wesker sees what they are doing and orders "reverse polarity" on the turbine so they will be sucked back into it. Saying "reverse the airflow" or "reverse the turbine" would have made more sense, so it's likely just a nod to this trope in sci-fi.

  • Discworld:
    • Subverted in one of the fiction-chapters of The Science of Discworld. After the Roundworld is transformed into a snowball (Ice Age), the Dean proposes (after four glasses of sherry) to "get Hex to reverse the thaumic flow in the cthonic matrix of the optimized bi-direction octagonate" to fix it. The Archchancellor replies that he would prefer a non-gibberish opinion.
    • There's a spell called the "Rite of Ashk Ente" which summons Death to you, in order to partake in his wisdom. Alberto Malich thought that if the spell makes Death go to you, then performing it backwards would make Death go away. However, he soon finds out that there is another way to consider the spell backwards: sending you directly to Death (which, oddly enough, worked out pretty well for him).
  • Edison's Conquest of the Moon (1898) describes the reversing of polarity nine separate times.
  • One of the villains in the second Firebird Trilogy book is trying to find out how to forcibly reverse the polarity of a telepath's epsilon field. When a reversed-polarity epsilon carrier is superimposed on a normal carrier through mental access, it releases a massive surge of psychic energy, temporarily boosting both psychics to stunning levels of power. The titular Lady Firebird turn out to have a mutation that gives her a naturally reversed epsilon carrier.
  • Robert A. Heinlein's Yellow Peril novel Sixth Column features two kinds of Reverse Polarity used with the book's race-specific-Death Ray. The first way turns it in to a health-ray (don't think too hard about that), and the second way lets you use it on non-Asians.
  • In the Star Trek: Voyager novel Ragnarok, the aliens of the week are using shields that automatically reverse polarity whenever something is shot at them. The crew figures out that they can shoot the shields and then reverse polarity so their next shot will ignore the shields.
  • Star Wars Legends: In the novelization of Revenge of the Sith, Obi-Wan uses the Force to "temporarily reverse the polarity of the electrodrivers in [Grievous'] mechanical hands," forcing them to open and drop his and Anakin's lightsabers. He later uses the same trick on Anakin's artificial hand.
  • Kemren the "Purple Mage" in Thieves' World generated mana by means of waterwheels. It gave him a lot of extra power, but running those waterwheels backwards was enough to beat him.
  • In Sandy Mitchell's Warhammer 40,000 novel Scourge the Heretic, the jury-rigged device to keep a machine going "reversed the polarity of the neutron flow" (a clear Shout-Out to Doctor Who).
    • Considering the amount of tropes invoked and subverted in the book and the fact that the above-mentioned machine simply didn't work due to the tampering, this can probably be considered deliberate.
  • Rhythm of War: In this book, Navani discovers how to invert the polarity of any of the three Light by singing the corresponding "opposite tone" (the exact musical inverse of that Light's native tone). This "inverse Light" violently annihilates the corresponding true Light (much like matter and anti-matter), making it capable of utterly and irreversibly annihilating creatures of Investiture such as spren and Fused.
  • In Captain Underpants, when Melvin has accidentally fused himself with a robot and his own snot, George suggests putting the batteries of the Combine-o-tron in backwards. Melvin scoffs at his suggestion and decides to spend six months building a Separatron, but Melvin's parents try switching the batteries at the end of the book and it works (though Melvin and Mr. Krupp suffer a "Freaky Friday" Flip).
  • Early on in Lux, Jax figures out how to reverse the polarity of the "motivator" fabrials that mimic the powers of Epics. Reversing the polarity always causes a motivator to produce an effect that is in some way the opposite of that motivator's normal effect, but what the "opposite" of any given effect turns out to be can vary from the obvious (inverting a motivator that attracts water causes it to repel water, inverting a motivator that grants Super-Speed causes it to slow time around the user) to borderline Moon Logic (inverting a motivator that lets you see an object's entire volume instead of just its surface creates a motivator that converts three-dimensional objects to two-dimensional drawings).

    Live-Action TV 
  • Batman (1966):
    • In the season 1 episode "Better Luck Next Time," Batman reverses the polarity on his belt communicator in order to create an ultrasonic signal to drive away a tiger.
    • In the season 2 episode "The Bird's Last Jest," Batman reverses the polarity on two electric cables leading into a pool. This causes the metallic chest containing Chief O'Hara to levitate out of the pool instead of shocking him.
  • Paul Chuckle is always using this in ChuckleVision. At one point he reverses the polarity of a UV light (which helps plants grow) to shrink his brother Barry.
  • CSI: NY:
    • Used realistically in the season 1 finale, "What You See Is What You See," had Mac actually "reverse the polarity" to show Stella the color-changing ink used by the counterfeiters.
    • Adam does it again in season 5's "The Triangle" to show Lindsay how a magnetic device used in an armored car heist works.
  • Doctor Who:
    • The two times Jon Pertwee used the exact phrase "Reverse the polarity of the neutron flow" are in "The Sea Devils" and the 20th anniversary special "The Five Doctors". note 
    • As mentioned above. After a long time of deliberately not using the phrase, the Tenth Doctor once said it as a Continuity Nod.
      The Doctor: Really shouldn't take that long just to reverse the polarity. I must be a bit out of practice.
    • An incident with the Fourth Doctor about a chicken turning into an egg, and it wasn't a failure — he was making a point about the technology. It Makes Sense in Context.
    • Doctor Who also provides a rare example of polarity reversal not working. When Nyssa and Tegan are rapidly aged every time the Doctor tried to take off the ship in "Mawdryn Undead" due to ... something or other, the Fifth Doctor attempts to reverse the polarity of the neutron flow. This only causes them to de-age into children. Try not to think about it.
    • The Doctor's sonic screwdriver once had its polarity reversed to turn it into an electromagnet and draw back a heavy bolt.
    • Parodied in "The Almost People". An exact duplicate of the Eleventh Doctor, called a "ganger" is created. As he tries to process the Doctor's multiple regenerations, he goes a little weird; at one point he says the "neutron flow" variety, only a few lines later to say, using Four's voice; "Would you like a jellybaby?" and then, in Eleven's voice, "Reverse the jellybaby of the neutron flow!"
    • "The Day of the Doctor" has a big love letter to this trope. The Tenth and Eleventh Doctors try to do this at the same time to a time vortex, only for nothing to happen. The Tenth realizes that their screwdrivers are cancelling each other out: "I'm reversing it and you're reversing it back again...we're confusing the polarity!"
    • The use of the phrase was lampshaded in "The Girl Who Died" when the Twelfth Doctor actually claims that the phrase is meaningless.
    • "Hell Bent": When Clara finds out the Doctor has a device he's planning to use to erase her memory of him so that she can survive without the Time Lords finding her, she uses his sonic sunglasses to reverse the polarity so that it would affect him instead. When she tells him this, he realizes what a horrible thing he was about to do, but someone still needs to get hit with the device (because Clara and the Doctor are too dangerous to travel together). Since he's not actually sure if she succeeded in reversing the polarity, or if that would even do anything to the device, he suggests they both hold it and activate it, and let karma take its course. They do, and the Doctor has all his memories of Clara erased. He still remembers their adventures, reconstructing them from the "Clara-shaped hole" in his memories, but he fails to recognize her even when he meets her again. He is then able to move on without her.
    • "It Takes You Away": While the Doctor is sonicking the mirror portal, Yaz suggests she reverse the polarity, a suggestion the Doctor is delighted by.
  • In Emergency!, there is a factory worker with his arm caught in a feeding hopper of a machine and Dr. Brackett is rushing over to have it amputated to save his life. However while he is en route, the paramedics come up with a better idea: they work with the factory's engineers and rewire the machine to make the hopper work in reverse to free the worker instead. Just as Dr. Brackett arrives, the modification is finished and they are able to free him instantly.
  • Eureka solves a lot of problems this way.
    • Blatantly and explicitly so in the season 3 finale, where Carter must reverse the polarity of a device causing a magnetic anomaly that threatens the whole world, before it gets blown up by the Martha drone sent by Zane and Fargo.
  • In Fringe, a device which creates wormholes had its polarity reversed, which, of course, reversed the flow of the wormhole!
  • Spoofed in Kyle XY, when Josh tells a drunken Kyle to sober up by "reversing the polarity of his liver". Kyle goes offscreen, vomits, and comes back sober, saying that he took Josh's advice. It's hard to say whether he was joking or not.
  • Reversing the polarity of the Lexx's main drive causes an EMP of sufficient power to fry any circuits on or near the ship.
  • The Man from U.N.C.L.E.
    • In the second season episode "The Minus-X Affair", THRUSH scientist Lillian Stemmler is developing a pair of drugs. One ("Plus-X") heightens the senses of the recipient to an almost superhuman degree. The other ("Minus-X") is intended to incapacitate its victims. And how is Minus-X made? Obviously:
      Arthur Rollo: Yes, yes. I know what it's supposed to do. But, uh, how about the other one? The Minus-X?
      Lillian Stemmler: No problem. To get the Minus-X drug, I only have to reverse the chemical processes inherent in the Plus-X.
    • In "The Moonglow Affair", Solo and Kuryakin are poisoned with deadly radiation from a THRUSH ray gun. To cure them, substitute agent April Dancer must obtain the ray gun so that UNCLE scientists can reverse the polarity and shoot our heroes with it again.
  • Mighty Morphin' Power Rangers:
    • In the episode "Opposites Attract", the magnet-themed monster did this trick to sabotage the Rangers' weapons. He could also cause a "polarity switch" for individual people and Angel Grove as a whole. What this actually meant apparently involved a Screen Shake and some storm effects.
    • In a later episode, Zordon commands Billy to reverse the polarity of a cloud. To make it rain. And it works.
  • On Moonlighting David and Maddie once claimed to have reversed the polarity of the viewers' TV sets, so they can register applause in order to vote on one of two ideas to make the show better. Maddie's idea was insightful drama, meaningful dialogue, and intelligent plots. David's was fanservice. Guess which won.
  • One of the earliest television uses was in a 1965 episode of My Favorite Martian. Martin tries it as he repairs the transmitter that works with his antennae.
  • Similarly, the cast of Mystery Science Theater 3000 once attempted to shake an alien monster off the satellite with a polarity-reversal maneuver; theirs involved an actual car battery and jumper cables. Also, the alien liked it.
  • The Outer Limits (1963): In the episode "The Borderland", during the experiments to reach the other dimension, the scientists reverse the polarity of a magnetic field in order to send objects to the other side and reverse them. A series of preliminary orders from the lead scientist to the technicians readying the reversal (that sound more like a magical incantation than technobabble) leads up to a final, dramatic "Polarity... REVERSE!"
  • Strangely enough used in Person of Interest, which normally takes the trouble to get its tech right. Apparently this is all you need to do to turn a speaker into a microphone. While any speaker can in fact be used as a crude microphone (Yamaha NS-10 speakers are often reversed into kick drum microphones in recording circles), all you have to do is plug them into a recording device. No "polarity reversing" is necessary, which is good, because speakers and microphones are alternating-current devices with no fixed polarity.
  • Power Rangers Dino Fury: "Crossed Wires" — The Rangers find Sleep Devices that have caused people to fall asleep. The Blue Ranger dismantles one by bypassing the capacitor, while the Green Ranger has trouble and instead causes a deeper sleep in the Black and Gold Rangers. The Blue Ranger then realizes he needs to reverse the polarity to wake the two rangers up, and teaches the Green Ranger how electronic circuits work in the process.
  • Power Rangers RPM: "Ranger Blue" — Flynn can't morph due to a bug in his morpher that has led to an energy buildup. While Dr. K is unable to work out a solution, Flynn realizes that he can discharge the energy by simply morphing with his activator chip in backwards.
  • Reversed polarity is apparently how the anomaly-locking device on Primeval works.
  • In one episode of Silversun, channeling the Star Runner's neutrino scanners allows Zandie to shoot down an entire pulsar to save Pancha's life.
  • In Sliders Season 2, Episode 3: Gillian of the Spirts, Quinn (via a spirit medium, the titular Gillian) instructs Arturo how to fix the timer by reversing the polarity.
  • In Smallville, a villain with magnetic powers pushes a car at Clark Kent. Clark catches the car, then uses a live wire to electrify it. Somehow, this causes the villain's "repel" to change to "attract", and he's pulled into the car and defeated.
  • Space: 1999: in "Black Sun", the Bergman invents an innovative protective shield which reverses the 'pressure' of the Black Sun, instead of just negating it. How reversing differs from negating is not explained.
  • Space Cases: Catalina is having some trouble with the concept from her textbook. "Reverse the polarity of the neutron flow? What's that all about?" Aren't shout outs fun? Later in the episode, she sarcastically suggests reversing the polarity as a solution to their problem... and is shocked to find out that it actually is a legitimate solution.
  • Stargate SG-1:
    • Lampshaded when Carter's analogue on the parallel show Wormhole X-treme! attempts to solve all problems by reversing the polarity.
    • Carter did actually get her homemade naqadah reactor working by reversing the polarity of a trinium plate in "Learning Curve". "Reversing the polarity" sounded more professional than "aw shit I put it in backwards".
    • And in "200", during a Star Trek parody, Mitchell as The Captain asks Carter to reverse the polarity in an overly dramatic manner.
  • Star Trek: The Original Series
    • In the episode "That Which Survives", Spock orders Scotty to reverse the polarity of a "magnetic probe"; Scotty responds with an incredulous "reverse polarity?!"
    • In the episode "The Doomsday Machine", what finally gets the malfunctioning transporter to work is when Spock tells Scotty to "try inverse phasing", which is literally the exact same thing as reversing the polarity.
    • Likewise, in the episode "Obsession", Spock did this by "cross-circuiting to B". Still the same thing.
  • Star Trek: The Next Generation
    • In the episode "Time Squared", Lt. LaForge and Lt. Cmdr Data must literally reverse the polarity of a shuttlepod's power supply in order to activate its internal systems. Even adjusting the flow of power must be done in reverse from what they're used to. Of course, this is due to the shuttle being from six hours into their future. As they approach the moment when the shuttle was thrown back in time, the shuttle (and the 'Future' Picard) begin to respond normally.
      LaForge: What happened?
      Data: The polarity was not compatible.
      LaForge: Not possible, the connection is idiot proof.
      Data: The power requirements of the shuttle do not match the Enterprise. We are going to need a variable phase inverter.
      Later, after hooking everything up backwards:
      LaForge: By all rights, this connection should overload all the shuttle's circuits.
      It doesn't.
    • Episode "The Naked Now". Near the end of the episode the Enterprise crew needs an extra minute so they can repair their engines in order to escape an incoming object. Wesley Crusher reverses the power leads on the ship's Tractor Beam to turn it into a repulsor beam to push off another ship and give them the time they need.
  • In the Star Trek: Deep Space Nine episode "The Sword of Kahless", Worf suggests reversing the polarity to get past the force field protecting the title object. Dax says, "That helped a little".
  • Star Trek: Voyager:
    • In the episode "Drone", Janeway and Seven of Nine thought they could fry a Borg tractor beam by reversing the polarity of Voyager's phaser banks; it looked on-screen like it was about to work, and instead their own phaser array was knocked out.
    • In a variation of this trope, Voyager worked around a cloud of warp field-dampening neutron radiation by simply "inversing" the warp field. ("Fair Haven")
    • Memory Alpha records at least five instances in which this order was given on the show. In three instances, it was helpful; in two, it was not.
  • Star Trek: Enterprise:
    • Justified in the pilot when they reverse the polarity of magnetic latches to get a structure to come apart.
    • Used almost word for word by Malcolm Reed in the episode "Harbinger" when he reverses the polarity on the plasma coils to knock out an alien.

  • A popular Filk Song, "The USS Make Shit Up" by Voltaire, about the Star Trek series contains a similar line: "Bounce the graviton particle beam / Off the main deflector dish / That's the way we do things, lad / We're making shit up as we wish..."
  • A musical example is in Bob Carlton's — sorry, William Shakespeare's — Return to the Forbidden Planet. Before the show starts, the 'crew' or ensemble members walk out and instruct the audience in standard safety procedures for their flight (air masks will deploy from ceiling, use of cell phones will cause the ship to explode, etc.) and end with teaching the audience how to Reverse Polarity themselves — only for an emergency situation, which is highly unlikely, nigh impossible — by putting their hands on their heads and twisting their torsos and heads to and fro. In Act II, of course, polarity needs to be reversed ("But it's not logical!" "Damn your logic! I've got lives to save!") and the audience has to help.
    • As an added bonus, at the end of the show, as the crew and captain prepare to launch back to Earth (singing Born to Be Wild, of course), crew members announce that all is well by calling "Iambics functioning, Pentameters locked in, Hyperboles all off the scale!" and "R.S.C. jettisoned", in jokes all relating to Shakespeare's text, a conceit of language, and the Royal Shakespeare Company, respectively.
    • Many of the posters (and T-shirts) for the show consist of a warning sign saying "WARNING: Do not reverse polarity!", and the playbill warns of the dangers of polarity reversal in space. Note that it is never actually specified what the polarity is reversed on, just that polarity in general is reversed - which makes the joke even more tongue-in-cheek...
  • Pops up in the Rush song "Vital Signs".
    • A tired mind become a shape-shifter/everybody need a mood lifter/everybody need reverse polarity

  • Lampshaded nicely in the Doctor Who BBC Radio drama The Ghosts of N-Space, in which the Brigadier jokingly suggests that the (Third) Doctor "Reverse the polarity of the neutron flow", to which the Doctor replies that the Brig knows as well as he does that the phrase is meaningless.
  • The Torchwood radio play "Lost Souls" plays this trope entirely straight, with the world saved from disaster by reversing the polarity of the positron flow. CERN apparently approved the science, and the impression is the writer was delighted to have found a context where the phrase actually made sense.

    Tabletop Games 
  • According to the Star Trek d6 RPG sourcebooks, reversing the polarity in different ways on the main deflector array can create a low power phaser, force someone out of warp, and allow you to basically do ANYTHING you could think of. The Main Deflector Array; Swiss-Army Weapon of the Federation. In fact players are encouraged to come up with Techno Babble explanations for whatever it is they are trying to do.
  • Magic: The Gathering made a card for "Reverse Polarity"—any damage taken so far that turn from artifacts is retroactively added to your health total. Lampshaded, but most certainly not explained, in that the picture shows a mace acting as a Healing Shiv. Subsequently, they made "Reverse THE Polarity" for the Doctor Who Universes Beyond set, with a picture of - of course - Jon Pertwee.
    • The Yu-Gi-Oh! card game has a similar card: Rainbow Life.
  • Paranoia adventure "'The Yellow Clearance Black Box Blues''. The Maxwell-Effect Moleculokinesic Field Device is basically a Pyrokinesis gun (e.g. it acts like a flamethrower). 50% of the time it fires at reverse polarity and freezes the target.
  • Ghostbusters RPG adventure Hot Rods of the Gods. If a Ghostbusters fires his proton pack at Meera at the same time as Meera shoots at him with the red devolvo ray, it will reverse the polarity. The devolvo beam will affect Meera and the Ghostbuster will evolve into a superior being with increased intelligence and a large head.
  • Toon supplement Tooniversal Tour Guide, "Atomic Monster Theater" setting. Professor Doug Graves can reverse the polarity of his portable razor and create a vibration to drive a Giant Potato Bug back into its cave.
  • SPI's Universe. When a psionic navigator wants their ship to make a hyperjump, they concentrate on the destination and encode their thought patterns on a plate of magnetic monopoles. This reverses the polarity of the monopoles and causes the ship to jump to the chosen destination.

  • Return to the Forbidden Planet has an audience participation moment when it's necessary to Reverse the Polarity of the Klystron Generator. A highly dangerous procedure.
    • The UK touring version of the show included a large neon sign saying "Do Not Reverse Polarity" as part of the set. The cast would also mingle with the audience before the show began, giving instructions on how to perform the polarity reversal procedure.

    Video Games 
  • In Freedom Force vs. the Third Reich, Sky King gets his jetpack to work and saves the day after Bullet tells him to reverse the polarity on his neutrino pack (in other words the battery is in backwards). Bullet is from the future and knew this from reading the Sky King comic books written after Sky King got his jetpack to work, but causality sucks anyway.
  • It turns this is the way to reconnect Clockwerk's body in Sly 2: Band of Thieves.
  • In Ikaruga, polarity reversing is a gameplay mechanic. White polarity absorbs white bullets, and does double damage to black targets. Black polarity does the opposite.
  • Space Quest V: The Next Mutation has you "reverse the phase polarity of the interface grid" multiple times throughout the game.
  • Referenced in Final Fantasy VI. One of the final bosses uses an attack named 'R. Polarity' to reverse the characters. Characters in the front rank of the party are switched to the back, and vice-versa.
  • The video game adaptation of the Death Gate novels uses Magi Babble instead of Techno Babble, but the principle remains the same. There's no point to casting a spell that sets you on fire—but reversing the order of the spell runes casts it on your Doppelgänger.
  • Star Trek Online actually has a skill called "Reverse Shield Polarity" that causes energy weapons to increase the shields instead of damaging them.
  • At the end of Ratchet & Clank: Going Commando, Angela's anti-Protopet weapon turns the Protopet into a giant monster. When she attempts to figure out what went wrong, Clank points out that the battery is in backwards.
  • The Elder Scrolls III: Morrowind makes a reference to this trope in a certain mage's notes:
    By reconfiguring the polarity of the daedron fields, it is possible to manipulate and trace the streams in the following cases...
  • Your Mission Control in Vanquish says this word for word. But in that case, you're just trying to make a Humongous Mecha's giant energy cannon blow up.
  • Thaddius, a raid boss in World of Warcraft, is a Frankenstein's Monster Expy who uses electric attacks. During the fight, each character is randomly assigned a negative or positive polarity. Players from each group must immediately converge on opposite sides of the boss or they will cause significant damage to anyone nearby with the opposite polarity. Thaddius causes a Polarity Shift every 30 seconds, which again randomly assigns the negative or positive charges. This requires players to quickly reorganize or risk a Total Party Kill.
  • Dota 2 has a literal Reverse Polarity spell, which sucks opponents in and stuns them. This spell is used by a nonmagical rhino warrior from a tribal society. How or why Magnus can "alter the properties of matter" is not explained, although it's implied that his horn has something to do with it.
  • Excursion Funnels in Portal 2 can be configured to pull things towards them rather than push them away by reversing the arms.
  • In Out of Order (2003), you need to reverse the polarity of your music player by swapping two wires around. This makes the device play music backwards, uncovering some backmasked messages.
  • In Turok 2: Seeds of Evil, one of the missions for the last level is to reverse the polarity of some power cells that generate toxins, causing them to purify the water instead.
  • In Fate/Grand Order, characters with Madness Enhancement that are subjected to something that would give it to them again are instead more lucid than normal. This happens to:
    • Caligula in Lostbelt 5.2. Artemis afflicts him with Lunacy, but since it was her influence as Diana that drove him insane to begin with, being subjected to it again restores his clarity.
    • Brynhildr in Summer 5. A summer swimsuit form changing her class to Berserker stabilized her Yandere behavior toward Sigurd, and they're able to date mostly normally.


    Web Original 

    Western Animation 
  • The Amazing Spiez!. In "Operation The 50 Ft. Hacker" a device called the Super Soaker Supersizer has its polarity reversed to return Megan and Davey to normal size.
  • In Ben 10: Alien Force, this is required to get Ben's hand back from the Null Void after Sunder separates it from the rest of his body. Ben 10: Ultimate Alien's episode "The Eggman Cometh" featured doctor Animo who used a Devolution Device to turn chicken into Pterodactyl-like creatures, then admitted under intimidation by Gwen that this trope is the way to fix everything.
  • In Danny Phantom, Danny uses reverse polarity on the Fenton Thermos to close a ghost portal.
    Sam: I'm impressed.
    Danny: (makes muscles) With my strength?
    Sam: No. That you knew what reverse polarity was.
  • In Dexter's Laboratory: Ego Trip, all that takes to turn Mandark's stupid dystopia into Dexter's brilliant utopia is to reverse the polarity of the Neurotomic Protocore from negative to positive, with a button.
  • Filmation 1960's versions of DCU heroes:
    • Batman: In "How Many Herring in a Wheelbarrow?", to stop the Joker's Solar Mirror from melting Gotham City, Batman must reverse the polarity of the Master Electrodes powering it by hitting them simultaneously with Batarangs. Seriously.
    • Hawkman: In "The Twenty Third Dimension", Hawkman's "quazer" is converted into a Dimension Ray by reversing its polarity.
    • The Atom: In "The House of Doom'', the Atom reverses the polarity of a miniature nuclear battery to turn it into a Self-Destruct Mechanism.
  • Ghostbusters:
    • It happened in The Real Ghostbusters all the time, and probably plenty of other cartoons where people tinker with electronics.
      • A particularly memorable from The Real Ghostbusters occurs when some ghosts get their hands on a proton pack — the standard Ghostbuster weapon — and try exacting some revenge. With the Ghostbusters busy elsewhere, it falls to the Ghostbusters' secretary Janine to corral the ghosts. She does this by using tools at hand and no formal technical training whatsoever to reverse a second proton pack's polarity so it will neutralize the ghost's weapon. Then, with a small twist of a screwdriver, she re-reverses the polarity and uses the pack to capture the disarmed ghosts.
      • In another episode where the Ghostbusters are trapped in an old movie studio and about to be blasted with their own packs, Ray reverses the PKE Meter to send signals to ghosts. It causes ghosts of the studio's hero characters to appear and save the day.
    • In an episode of Extreme Ghostbusters, the team was faced with a ghost that multiplied whenever the proton packs were used against it. They overcame this by converting them into electron packs.
  • Shows up in the second G.I. Joe miniseries. Apparently, "reversing the polarity" of a pair of antennae just involves making the tips touch.
  • The Herculoids episode "Revenge of the Pirates". Zok does it with his laser Eye Beams to neutralize a force field.
  • Reversing the polarity of energy sources was one of the many plot convenient things Penny's computer book could do on Inspector Gadget.
  • IQ in the James Bond Jr. cartoon also reverses the polarity all the time. He once took control of the bad guy's helicopter with a simple remote control and some polarity reversal.
    • One episode had James himself perform probably the most ludicrous variation ever: In the beginning of the episode, James berates IQ for not fixing his digital watch which for some reason has started counting backwards. Later, James is locked in a room with a doomsday device set to go off. What does he do? He uses some pieces of wire to connect the watch to the device's timer and lo and behold, the watch starts working normally and the timer starts counting backwards.
  • The Adventures of Jimmy Neutron, Boy Genius: This is the solution to many a kink in Jimmy's machines, whether it makes sense or not.
  • Subverted in the Founding Zombies episode of Johnny Test. When Johnny uses a ray gun to bring the town's Founding Fathers back to life, Susan (or Mary) attempt to stop them by "reversing the polarity" of the gun. However, because they weren't completely alive, it did nothing. Johnny then had them "reverse the reverse polarity thing" to bring back their mothers and "save" the day.
  • In the Jonny Quest: The Real Adventures episode "DNA Doomsday", Hadji reverses the polarity of a DNA blob monster, turning it into mush.
  • Justice League: In "Secret Origins", Batman saves the day by "reversing the ion charge" on an alien ship (this also appeared on the first episode of Challenge of the Superfriends, and very similarly).
  • In the Kim Possible episode "Clean Slate", reversing the polarity on a memory-enhancement device causes it to erase Kim's memories.
  • In Megas XLR due to some wire changes, a weapon Coop uses accidentally gives the enemy reflective shielding. Then when he teleports the stabilizer out of the enemy robot it "reverses the polarity" of the shield making the opponents weapons hit itself.
  • An episode of the Men in Black: The Series had a super-power giving device. How do you undo its effects? To paraphrase:
    Alien 1 That would be very complex...
    Alien 2 Nah, just switch the wires in the bottom and use it again.
  • In The New Adventures of Superman, reversing the polarity of an electric charge transferred the powers of the episode's villain (and some other guy) back to Superman... After they got them from him through an electric shock.
  • South Park spoofs this (spoilered for squick, not actual plot spoilers): Cartman tries to "make Butters gay" by putting Butters' dick in Cartman's own mouth and taking a picture while he sleeps. Kyle and Stan tell Cartman that it makes him gay, and that he has to put his dick in Butters' mouth to reverse the gay polarity. Hilarity Ensues as while he doesn't succeed (and eventually realizes the others were messing with him), it does get Butters sent to a gay cure camp.
    • Also used in the episode "Cancelled", when Jeff Goldblume tries to reverse the polarity of the satellite in Cartman's asshole to figure out where the video is being sent.
  • Star Wars Rebels: In "Double Agent Droid", Chopper gets remote-hacked by an Imperial specialist. Hera pays him back by sending a massive burst of energy down the signal that he was using to control Chopper, blowing up his ship.
  • Superfriends 1973/74 episode "The Shamon U". Professor Shamon reverses the polarity on his giant electromagnet to repel the Batmobile instead of attracting it.
  • After Peach was accidentally transformed into a baby in an episode of The Super Mario Bros. Super Show!, Mario and Luigi got her back to her true age by reversing the flow of the Fountain of Youth.
  • In an episode of the original Teenage Mutant Ninja Turtles cartoon, "Enter The Fly", one of Shredder's evil schemes failed because Baxter Stockman "forgot to reverse the polarity of the neutron flow".
    • This is also how Donatello Deus Ex Machina's just about every other episode once all of the hilarity and wacky hijinks have been taken care of.
  • In The Tick Vs Reno, Nevada, Arthur is asked to reverse the polarity on a fish magnet. He finds two cables, blue and red, and a box with two similar a big label saying "Observe Correct Polarity. Use AC current", and two colored sockets. Arthur puts the cables in the wrong sockets, which seems to work, despite the obvious AC current issue.
  • In the very last episode of The Transformers, "The Rebirth, Part 3", the heroes derail Galvatron's plan and bring about a new Golden Age on Cybertron by reversing the polarity of Galvatron's rocket thruster.
    • And in "Call of the Primitives", Grimlock manages to stop Tornedron, an energy-eating being, by reversing his energy polarity (by throwing a switch).
  • Transformers: Prime uses this as the Autobots' method of blowing up Megatron's space bridge.
  • One of the Shen Gong Wu artifacts in Xiaolin Showdown is the Reversing Mirror, whose primary purpose is to invert the effects or qualities of other Shen Gong Wu. The demonstration show it making the Two-Ton Tunic as light as a feather, and it becomes prominent in one episode for being used with a shrinking Wu to enlarge objects.

    Real Life 
  • When a phone exchange fails to hang up on a land line, what do you do? Polarity Reversal!
  • A certain model of text message pager made by Motorola and Unication can be cleared from certain faults by removing and re-inserting the single AA battery in reverse to the normal installation for about 15 seconds, then removing it and installing it normally again.
  • Antimatter is normal matter with Reverse Polarity. The charges of protons and electrons are switched.
    • Every particle also spins into the opposite direction as their normal counterpart. Combining both gets you a real-life Yin-Yang Bomb.
  • Technical support sometimes ask people to reverse the wire. Of course, this is just a subtle way of making sure the customer has the danged thing actually plugged in.
    • A frequent cause of problems for remotes is their batteries having been put in backwards. In cases like these, literally switching the polarity (i.e. flipping the batteries around) will solve the issue.
  • Most mains-powered electronic devices with ungrounded (2 prong) cords have the neutral side of the cord connected to the case through a small capacitor. This provides a high-pass filtered path to ground on the case. The idea is to allow high frequency (typically RF) noise to be shunted to ground to prevent interference. Back in the days of non-polarized two-prong plugs, there was actually a benefit to reversing the plug to minimize noise. While the line is AC, it is not balanced. One side is grounded. If two audio devices are connected together, it is best if both cases are bonded to the same side of the mains wiring, ideally the grounded side. Otherwise a small AC current is induced in the shield of the interconnecting cables, which in turn induces a hum in the signal wire.
    • Ground noise actually pops up in the modern age, too. VGA cords conduct the damn crap, causing a ground loop (powerstrip-computer-cable-monitor-powerstrip). Monitor/TV noise (generally seen as "waves") is a classical feature. HOWEVER, the best way to fix this is to unground the computer, though you should then avoid touching metal parts of the chassis when plugged in, just in case.
    • In the days of the non-polarized two-prong plug, it was standard practice to have the high-pass capacitor on a switch in case the device (usually an amplifier) is plugged in backwards by mistake, so the path from live to neutral can be switched from one mains lead to the other. There's a reason the capacitors in these devices are referred to as "death caps" today, as reversed polarity has been implicated in the electrocution of singers and guitarists performing in damp conditions; it is now standard practice to rewire such vintage equipment with an earth bond to the chassis.
  • Reversing the Polarity of one speaker of a stereo sound system can have real effects. This happens because waveforms that are 180 degrees out of phase will cancel each other out. The listener will usually hear this as weakened bass response. Of course, this only works once: "My stereo sounded much better when I reversed the polarity on the left speaker. I can't wait to hear how much better it will sound when I reverse the right speaker too!"
    • This can also serve as a simple way of converting mono to stereo. Take a mono sound file, put your sound editor into stereo mode, then invert one of the channels. Quick surround sound. Mind you, it only works well with headphones, and if the resulting file is converted to mono again, silence will result.
      • Which is how noise canceling works. Two waves of 180 degrees will cancel each other out. Of course, this only works when the noise is more or less predictable.
  • Live sound reinforcement mixing consoles have a "phase" switch on every input which is really just a Reverse Polarity switch. This is often used when placing two microphones on the same sound source directly facing each other. If there are 2 microphones on a snare drum, one above facing down and a second below facing up, the engineer will usually Reverse Polarity on the bottom mic.
  • Converting a voltage follower with gain into a comparator with hysteresis is as simple as reversing the polarity of the amplifier.
  • In classical Chinese thought, if the Yellow River runs clear when it should be muddy, or muddy when it should be clear, or anything that should be Yin is Yang (or vice versa), it is considered an omen that the ruling dynasty has lost the Mandate of Heaven. Reversing the polarity, in other words, is an omen of DOOOM!
  • The Earth literally reverses the polarity of its magnetic field every couple of hundred thousand years. Scientists are not entirely sure what happens to life when it happens, but you can bet it will be the solution to somebody's problem.
    • Similary, the magnetic field of the sun itself rearranges itself so that at any point, in eleven years it'll be the exact same pattern, only backwards (and when it goes another eleven to twenty-two years, it gets back to where it started).
  • Diodes will work different depending on the direction of current.
    • Namely they will refuse flow in one direction.
      • That is, until the voltage is so high that they start to leak in the opposite direction. Diodes specially created to reliably and deterministically exploit this are called zener diodes. Quite useful when you need a precise voltage drop.
  • Thermal optics can also reverse polarity. Doing so switches between White Hot and Black Hot (Meaning that heat shows up as white or black, respectively.)
  • In an MRI, the spinning dipoles slowly come out of sync with each other over time, which makes it difficult to observe them. The solution is to use a burst of radiation to reverse the polarity of the spin, which means that all the dipoles will sync up again after the same amount of time between the initial burst and the polarity reversal. This process can be repeated until other sources of error eventually overwhelm the signal.
    • The NMR word for this is "echo". If the name for a given pulse sequence contains the word "echo" (e.g. Spin-Spin Echo, Quad Echo), you can bet there's a polarity reversal in there somewhere.
  • DC motors spin in the opposite direction when their power source is reversed.
    • Which is why electric cars can drive backwards despite usually having only a single fixed gear instead of a conventional gearbox: to drive such a vehicle backwards, you just reverse the polarity of the electrical current flowing through the motor.
    • Three-phase AC motors do the same by swapping two of the phases.
    • Not only that, but you can reverse the polarity of the energy flow: apply torque to the axle and it becomes an electrical generator. (Electric cars do this with 'regenerative braking'.)
    • DC model railroads work this way. You can turn the controller in two directions and send either a "positive" or a "negative" voltage to the rails. So in order to make a train run into the opposite direction, you can simply reverse the polarity in the controller. AC is much more complicated because it takes a relay built into the vehicle to reverse the direction.
  • Electrolytic coating. If the given polarity makes a metal condense on the electrode, the reversed current will dissolve it back.
  • Vascular plants get rid of air bubbles in the xylem by reversing the flow of water during the night.
  • In Capillary Electrophoresis, reversing the polarity is necessary to have your sample actually flow to the detector. Otherwise you just lost all of your sample and contaminated the buffer, And That's Terrible.
  • Most unlicensed games for the Nintendo Entertainment System stopped the console's normal reset cycle by using a charge pump to send negative voltage spikes on the data pins to freeze the lockout chip.
  • Brian May's custom-made guitar, Red Special, was made with Burns Tri-Sonic pickups that May had re-wound with reverse polarity.
  • In the old days of railroading, before the widespread use of brakes, one way to stop a steam locomotive was to put it in reverse and then reduce the steam. This would stop the engine faster than merely reducing the steam would.
  • Early experiments with supersonic flight revealed that, occasionally, the controls would start doing the opposite of what the pilot commanded (i.e. pulling back on the stick would cause the nose to drop rather than rise, or simply nothing (this was an issue with the Bell X-1 in particular, as a shockwave formed on the elevator hinge, nullifying its control authority completely). This was caused by either the wings and stabilizers warping under the unprecedented stress, or supersonic shockwaves interacting with the control surfaces in unpredictable ways. The initial solution was just to use opposite inputs (i.e. pushing forward on the stick to go up) until you got the airplane slow enough that the controls would respond normally again. A more permanent solution was to design supersonic planes with stiffer wings and much larger control surfaces.
  • The Boeing 737 used to have a design flaw that, under certain circumstances, caused the rudder to move the opposite direction of what the pilots would command. That flaw resulted in two plane crashes (United Airlines Flight 585 and USAir Flight 427) and the temporary loss of control of a third (Eastwind Airlines Fligh 517).
  • Analog synthesizers are often voltage-controlled, especially those with no patch memory including modular synths. Reversing polarities is common.
    • Oscillators in general produce something similar to AC, but with different waveforms.
    • Envelopes can be polarity-reversed to have a negative effect on whatever they control. Yamaha even built envelopes that could reduce their voltage beyond zero to negative.
    • There are Eurorack modules for that.

Alternative Title(s): Polarity Reversal, Reverse The Polarity, Reverse Polarities