How to determine polarity of electric wires.
July 27, 2011 12:38 AM   Subscribe

I have two DC electric wires: one is positive, one is negative. Without buying anything, how can I tell which is which?

Assume it's .5amp and 12 volt. This actually isn't just a theoretical situation though: I snipped the DC end of an AC adapter and now I can't remember which was which. The wires are identical. My plan is to use it as a car battery tender/charger, but I don't want to put it on backwards.

But also I'm interested in what cool science tricks there might be for detecting positive/negative. Go science tricks!
posted by brenton to Science & Nature (16 answers total) 6 users marked this as a favorite
Without using any other kit I don't think it's possible. If you're willing to splash out you could use a diode and bulb; a diode only allows electric current to flow in one direction. Or you could use the power supply to do some electrolysis and see what is formed at which electrode.
posted by alby at 1:02 AM on July 27, 2011

Take it to a friendly auto parts shop, garage, hobby electronics shop, computer shop, or any other place they're likely to have a multimeter, and ask them to do it for you.

(Also take a bit of red tape to mark the positive lead so you don't forget on the way home..)
posted by Ahab at 1:03 AM on July 27, 2011

Best answer: In a darkened room, hook one lead to the negative terminal of the charged battery. quickly brush the other lead against the positive terminal (don't leave the circuit completed for any substantial amount of time). Try reversing it. The one that made less sparks is the right direction. The momentary contact probably won't ruin anything. :D

Be aware, that you aren't going to be able to charge a car battery with a 12V dc adaptor; when the battery's voltage is as low as 12, it's basically already dead to the point of being permanently damaged. You'd ideally charge it with 13.2-14.4V.
posted by aubilenon at 1:03 AM on July 27, 2011 [1 favorite]

An LED and a 1k or so resistor, in series. If the LED lights up, the long wire is positive. You might be able to do without the resistor, at the risk of frying the LED.

(Okay, those are parts, but it's maybe 50 cents worth.)
posted by neckro23 at 1:08 AM on July 27, 2011

Best answer: You use a compass or magnet on a string, make a coil of the wire, and use the left hand rule.

Bonus points: making your own magnet from scratch using the same coil and a leftover nail. Finding north (so you know which end of the magnet is which) is an exercise left up to the reader.
posted by Pinback at 2:44 AM on July 27, 2011 [3 favorites]

Best answer: If you're building a car battery charger and you're not MacGuyver, you should probably have/get a multimeter. They're not expensive (very low end for $10, good one for under $50). It will help you with other charger-building tasks as well.

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 (or a stick, piece of chalk, whatever, ideally an iron rod but you need a working charger now because Soviet agents are only minutes away! Whatever's at hand, just make sure you mark one end)

1) Wrap the wire around the pencil 20-30 times (or more, but Boris is at the door!). 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. **

Even easier solution: just connect the charger randomly and plug it in. So long as you have two batteries (also two garages and a good medical plan) there's no quicker/cheaper way to find out.

Really, buy or borrow a multimeter.

* Suitable? I am not your mechanic/engineer and I don't recommend you try this at all. You are not being pursued by Soviet agents. Don't experiment if you don't know how to calculate what's safe. Even if you do, don't. No need to invent new ways of setting your garage on fire.

** Assuming that at this time of night I correctly remember both the right hand rule and how compasses work. It also works if I'm wrong about both.

posted by justsomebodythatyouusedtoknow at 2:45 AM on July 27, 2011 [11 favorites]

I even previewed. Late by thirty seconds?

For my next trick: how to build a time machine using only insulated wire, a chewing gum wrapper, a Y-shaped stick and a stainless steel sports car.
posted by justsomebodythatyouusedtoknow at 2:51 AM on July 27, 2011 [1 favorite]

Best answer: Did someone say science tricks?

By electrolysis
Get some water with enough salt dissolved in it to allow a current to flow; maybe a tablespoon of salt into 500ml water.

Dip the wires into the water a few cm apart. After a minute or two (it might be faster or slower, I'm guessing based on the voltage), bubbles should start to form at each end. The -ve electrode should generate roughly twice as many bubbles as the +ve.

You're getting hydrogen gas at the -ve, oxygen at the +ve. You can also try capturing the gas by putting each electrode under an inverted, heatproof container that's submerged and initially full of water. CAREFULLY putting a glowing ember (e.g. the tip of a just-extinguished long match) into each gas will help you identify them: Oxygen (from the +ve wire) will brighten the glow and/or reignite the match, hydrogen (from the -ve electrode) will give you a tiny explosion with a characteristic "squeaky pop". This is safe enough for high school students to do, but don't point anything at your face and keep your fingers at least 8 inches from the gas when you try to light it.

By measuring magnetic field
Hook up small bulb or something to the wires, so you can have a current flowing without short-circuiting the power supply. Or just connect the wires via a resistor or a rusty paperclip or something. A graphite pencil lead would probably be ideal.

Hang one of the wires vertically, away from the other, then put a compass on a shelf immediately next to it. Switch the power supply on. If the current is strong enough (I have no idea, sorry), the compass should move to align with the magnetic field in the wire. Look where the compass' North needle is now pointing. The use the right hand rule to work out which way the current is flowing.

First, look at the side of your index finger and imagine (or draw) an arrow from your palm running up to your fingertip. Next, curl your fingers around an imaginary wire into a fist. Now stick your thumb up so you're making a thumbs-up sign. Your thumb represents the direction that current is flowing through the wire. Your curled fingers represent the magnetic field around the (imaginary) wire in your hand. At any given point around that circle, the arrow along your index finger (running from palm to fingertips) shows which way the "North" end of an adjacent compass needle would be pointing:

Now, look at the real dangling wire with the real compass needle next to it. Simply keep your hand in "thumbs up" mode and twist your wrist until the arrow on your index finger points the same direction as the arrow on the real compass. Assuming you've avoided any major dislocations, your thumb is now pointing in the direction of current flow through the wire, which tells you which terminal is which.
posted by metaBugs at 2:53 AM on July 27, 2011 [2 favorites]

Be aware, that you aren't going to be able to charge a car battery with a 12V dc adaptor; when the battery's voltage is as low as 12, it's basically already dead to the point of being permanently damaged. You'd ideally charge it with 13.2-14.4V.

Repeated for emphasis. Charging voltage is higher than the nominal cell voltage for a lead-acid battery. 13.8V is pretty much what you want to aim for for a float-charger. You absolutely need a multimeter for this, so forget the MacGyver strategies and just go buy one.
posted by Rhomboid at 4:54 AM on July 27, 2011 [1 favorite]

Look more carefully--the wires are likely not identical. There will be a white or gray line, or a raised rib along the outside of one of the wires. In some cases the wire itself is different--copper vs. silver color perhaps. Do you still have the jack end you snipped? In that case you can look on the adapter for a polarity symbol. Then carefully cut the plastic off the jack until you can see which wire goes where. Best thing is to get a multimeter, though. They are cheap and very useful. You often find that the output is higher or lower than indicated on the adapter.
posted by weapons-grade pandemonium at 8:21 AM on July 27, 2011

Response by poster: Yay SCIENCE. Also MacGyver. The electrolysis trick looks the simplest so far, since I don't have a compass, and if I'm buying things, I might as well just get a multimeter. (Which I always thought was called a Vold-o-meter, lol.)

Side note regarding those discouraging a 12v adapter: does this discouragement apply also to a battery tender (e.g. leave it plugged into my battery all summer while I'm away)? I can probably find a adapter pretty close to 13.8v around here somewhere and use it for charging overnight instead of asking for a jump in the morning. (Yes, at the risk of explosions.)
posted by brenton at 8:57 AM on July 27, 2011

a couple people already have the right idea with needing more than 12V to charge your battery.

also, spend 10 bucks on a cheap multimeter from walmart or harbor can be a very useful tool to have around the house.
posted by mikesrex at 9:08 AM on July 27, 2011

Best answer: If you try to charge your battery with a 12v, you will likely ruin it. Read up on battery maintenance. A 12v adaptor will make your battery worse, not better.
posted by aubilenon at 9:38 AM on July 27, 2011

Response by poster: Thanks for that link, aubilenon, very informative!
posted by brenton at 11:15 AM on July 27, 2011

Which I always thought was called a Vold-o-meter

That's Volt-Ohm meter. Sometimes also used to be called an AVO meter [Amp-Volt-Ohm]. But these days generally just called a multimeter.
posted by HiroProtagonist at 9:32 PM on July 27, 2011

Response by poster: *volt-o-meter
posted by brenton at 2:02 AM on July 28, 2011

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