I came across this question about how to figure out the polarity of a cell phone charger without a meter. This made me think of what methods you could use to figure out the polarity other than using a meter or any device that is better than a meter; and I thought it would be an interesting question for my first question here.

My answer was that you could use a LED or a diode to figure it out. Or you could use salt water, as long as the voltage is high enough (over 2 volts, I think,) you could place both leads into heavily salted water and the negative lead will produces more bubbles and corrode faster.

Another ideas I thought of would be to use a linear regulator and see if you destroy it (testing if you destroyed it would require a meter or at least a circuit where it was working originally.)

What other ideas/options can you think of? Like electromagnetic methods or even blowing up capacitors...


As @Juancho pointed out, when I said without a meter I meant without a multimeter (or a more advanced device.)

Edit 2

Since this is basically a thought experiment, feel free to make the voltage and current whatever you want, but try to point out the range if it matters. For example @DeanB's neon answer required ~70 volts.

There are some great answers so far, thanks everyone! I'm sure there must be more basic chemistry options, unfortunately I never took chemistry in school.

Edit 3

Building a meter is also a valid answer. In fact there really isn't a wrong answer, other then saying, "use a multimeter."

This is becoming quite cool, some very interesting and unique answers! Thanks everyone!

Another Answer

I found this elsewhere and thought it was interesting, however @Juancho already posted a similar answer.

How MacGuyver might find out:


  1. some insulated wire
  2. a compass
  3. a suitable resistor
  4. a stick of chewing gum
  5. a pencil


  1. Wrap the wire around the pencil 20-30 times. Looking at it from the eraser end, the wire should be going around counterclockwise.
  2. connect wire #1 of your adapter to the clockwisemost end of the coil (still looking from the eraser end). In series, connect the resistor to the other end of the coil. Connect the other end of the resistor to adapter DC wire #2.
  3. chew gum (save the wrapper in case you need to defuse a bomb later in the afternoon)
  4. plug in the adapter
  5. hold the compass near the eraser end of the pencil 6) if the compass points toward the eraser end, #1 is the negative wire. If it points away, #1 is the positive wire.
  • 1
    \$\begingroup\$ Yey, fun question. +1 \$\endgroup\$ – Vorac Sep 29 '12 at 7:27
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    \$\begingroup\$ Basically what you're asking is how would McGuyver figure out polarity: en.wikipedia.org/wiki/MacGyver \$\endgroup\$ – jippie Sep 29 '12 at 7:41
  • \$\begingroup\$ Anything you measure it with is a meter because of what it is being used for, do you mean without using a multimeter? Or maybe you just mean a voltmeter? \$\endgroup\$ – Kortuk Sep 29 '12 at 13:14
  • \$\begingroup\$ @jippie Exactly! \$\endgroup\$ – Garrett Fogerlie Sep 29 '12 at 15:25
  • \$\begingroup\$ @Kortuk, I mean without any device that is intended to be used to test polarity. It's just a fun thought question looking for unique answers. \$\endgroup\$ – Garrett Fogerlie Sep 29 '12 at 15:26

12 Answers 12


1: You can pass current from your source through a wire and check the direction of the magnetic field with a compass.

But, then, a compass is a meter also.

2: Make a circuit by connecting in series a battery, your source and a resistor. The configuration that dissipates more heat on the resistor (use your finger as a heat probe) is when DC source and battery are placed with the same polarity ( +DC- +BATT- R ).

  • \$\begingroup\$ +1 Great answers! #2 is something I wouldn't have thought of. Also, by a meter, I meant multimeter or any advanced device that is meant to measure polarity like a O-scope. I'll update my question to make that clear. \$\endgroup\$ – Garrett Fogerlie Sep 29 '12 at 2:27
  • \$\begingroup\$ I had the same #2 idea, but with a light bulb. \$\endgroup\$ – jippie Sep 29 '12 at 7:54
  • \$\begingroup\$ For #2, couldn't you simply go source - diode - low-value resistor? The resistor would get hot almost instantly, but only if the current actually flows. :) \$\endgroup\$ – exscape Sep 29 '12 at 8:21
  • \$\begingroup\$ I think the implied meter is a multimeter, but that might not be the intent. \$\endgroup\$ – Kortuk Sep 29 '12 at 13:13




Reverse the battery or the diode.

[edit] @perilbrain: You're right, I was only considering the low voltage such a charger might have and the suggestive "BIG_R" and "small_r", not that others might come and ...just try it "for the laughs" (or rictus).

  • \$\begingroup\$ That sir, that is funny. I also was thinking of ways in which I could get the user to electrocute themselves, always a good call. \$\endgroup\$ – Kortuk Sep 29 '12 at 13:15
  • \$\begingroup\$ You should put warning notice above while suggesting such techniques. There is no age limit on joining this community. \$\endgroup\$ – perilbrain Sep 29 '12 at 14:40
  • \$\begingroup\$ That's a good one! \$\endgroup\$ – Garrett Fogerlie Sep 29 '12 at 15:17
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    \$\begingroup\$ I've always wondered what the schematic symbol for drool was! \$\endgroup\$ – HikeOnPast Sep 29 '12 at 15:55

For cell phone charger which it's voltage is low i recommend use led+resistor.If you connect V+ and V- like below your LED will turned on and if not it will be off. LR1 generate your current. So you should read your LD1 current requirement and then calculate LR1. I_led=(V+ - V-)/LR1 enter image description here

Breakdown voltage of LED is very low, so a protection should be considered for reverse biasing.Diode D1 is used for protection if reverse voltage exceeds 0.7, D1 will conduct. So LD1 in reverse voltage is off and it is safe for reverse biasing.


1) If supply > ~70V , apply the source to a neon bulb and series resistor. The negative terminal will glow:

enter image description here

2) If high current, pass through a length of wire near a magnet of known orientation. Based on whether the wire levitates or attracts toward the magnet (right hand rule), the direction of current flow can be determined.

  • 1
    \$\begingroup\$ THat's a hell of a cell phone charger!! ;) \$\endgroup\$ – kenny Sep 29 '12 at 11:30
  • \$\begingroup\$ @kenny What, your cellphone charger does not use 100Ah batteries? \$\endgroup\$ – Kortuk Sep 29 '12 at 13:15
  • \$\begingroup\$ Never would have considered a neon bulb! \$\endgroup\$ – Garrett Fogerlie Sep 29 '12 at 15:21
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    \$\begingroup\$ It can be a pain getting 480V pulled to your house for one of the new "Level 3" chargers, but my phone sure does charge fast! \$\endgroup\$ – HikeOnPast Sep 29 '12 at 15:58
  • \$\begingroup\$ @HikeOnPast if "charge" means an explosive charge. \$\endgroup\$ – user253751 Feb 5 at 12:49

Attach a wire to each electrode. Insert the wires into a potato and note which lead causes the potato to turn yellowish. You have found the negative electrode. This technique was used by Thomas Edison.

  • \$\begingroup\$ safe, convenient, works. best answer so far if you just need a quick test (unless, of course, you lack a potato). THANKS! \$\endgroup\$ – Shaun Wilson Jul 15 '14 at 20:42
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    \$\begingroup\$ This is why I always carry a potato. \$\endgroup\$ – Ian Bland Sep 24 '16 at 23:25
  1. Open the casing, then look for markings on the PCB or search for a polarized output capacitor.
  2. Write a question on http://electronics.stackexchange.com or other electronics forum.

  3. Type the partnumber in Google.

  4. Use a polarized piezo buzzer like in the image below. Either it works (polarity is correct) or it doesn't (polarity is incorrect).
  5. Check the label on the power supply, most of the time there is some indicator on it, or find a different power supply.
  6. Power one of your little brother's electronic toys from it. Brother happy => polarity correct; Brother crying over smoking toy => bad polarity or too high voltage. In latter case try to find a different toy.

piezo buzzer

  • \$\begingroup\$ I like #5 and #7! \$\endgroup\$ – Garrett Fogerlie Sep 29 '12 at 15:20

With a known source:-

Using Motor:-

Take a batery of known polarity and see the direction of rotation.Try the same with a voltage source of unknown polarity.If rotational direction is same then unknown has polarity same as known else opposite.

Using Electromagnet:-

Wind the wire around two iron bars each connected to either one known or another unknown voltage source.Bring them closer and notice attraction or repulsion.If both attract then they have opposite polarity else same.

Using Bulb:-

If the below circuit glows then polarity is same else opposite.(Theoretical circuit, may burn the wires and components, use appropriate resistors)

enter image description here

Using Tongue(Highly Discouraged):-

Modify previous arrangement by removing the bulb and the upper connector. Now touch your tongue to both upper terminals.Shock implies opposite polarity.


To add to the already comprehensive list, connect a loudspeaker (not a tweeter or midrange, a woofer). If the cone moves out, the positive output is connected to the positive tab (perhaps marked with a red dot or plus sign) on the speaker. If the cone moves in, the negative output is connected to the positive tab.

Naturally, you would want to try this only with known low-voltage supplies (a cell-phone charger certainly qualifies) and you wouldn't want to leave the speaker connected to the supply for any length of time. Just a touch to see which direction the cone moves.


(Discouraged if you don't know what you're doing)

  1. Assuming you have a decently powerful enough source, you can hook up a polarized electrolytic capacitor. If it leaks the magical grey smoke and/or pops, you hooked up the terminals backwards or your source exceeds the voltage rating of the cap. Otherwise, your power source is too wimpy to produce a dramatic effect, or the polarity is correct.

  2. Get a tub of a baking soda aqueous solution. You can use salt, but be careful about other noxious chemicals that will be created. Strong acids/bases work well too, but most are hard to get as a regular consumer. Stick two test tubes with open end down into the solution, both filled with the aqueous solution. Stick on terminal inside one test tube and the other inside the other test tube. After the test tubes have a good amount of gas collected, take a match, light it, and stick it in a test tube. The test tube which produces a noticeable wiff/mild explosion contained hydrogen and is the negative terminal.

  • \$\begingroup\$ I want to point out that with #2 the negative will also produce more bubbles and corrode faster (unless I have that backwards.) Thanks for the detail! \$\endgroup\$ – Garrett Fogerlie Sep 30 '12 at 0:33
  • \$\begingroup\$ Yeah, it should produce more bubbles by because you're creating twice as many hydrogen molecules as you are oxygen. But it's easier and funner to use the match test :) \$\endgroup\$ – helloworld922 Sep 30 '12 at 2:23

You can get a jar and fill it with vinegar and add salt until the salt no longer dissolves. Then connect one wire from the unknown source to a piece of copper (you could use a bundle of copper wire but it would be better if you use a piece of copper like a pcb board.) Now connect the other wire to a non-copper key. Place both the copper and the key into the solution, don't let them touch each other.

After a while (~30 mins, but it will depend on the metals, acidity, current and voltage,) you can take the items out of the solution.

If the key has a copper finish on it, and the copper piece is dull or missing copper; then the key's wire is the negative feed and the copper's is the positive.

Otherwise the key's wire is positive and the copper's is negative; and you may notice the key is dull and the copper has some of the finish on it that the key originally had.

Here are some pictures of what it will look like if the key is negative.


enter image description here


enter image description here

For anyone who doesn't know, this is called electroplating and is used to chrome plate metals, or even to get gold to the .9999% purity. Basically the metal on the positive side dissolves into an ionized state and clings to the metal on the negative side where they convert back to the metallic state.

This is similar to my second answer that I posted in the question (the electrolysis one, with the negative lead corroding and bubbling creating hydrogen.)


Use a small DC motor. 'Calibrate' on a source of known polarity and mark the motor tab that gives clockwise rotation when positive.

  • \$\begingroup\$ I feel like this one has been mentioned, but +1 anyway incase it hasn't. \$\endgroup\$ – Garrett Fogerlie Nov 5 '15 at 3:37

Use a small electrolytic capacitor of higher voltage as the supply. If you connected it with the right polarity nothing will happen. If you connect it with the wrong polarity it will explode.

(This is given in jest. Don't do it!)


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