# Picking Wire Gauge

Q:
Ive been reading your posts on electromagnetism and have a question of my own. Suppose you were constructing an electromagnet using a car battery, if you used a small gauge wire to optimize the amount of wire turns around the core, wouldn't the wire just vaporize? how would you make an electromagnet with a car battery?
- matt (age 25)
binghamton, ny, usa
A:
You're right to worry about that. However, for a given volume of winding, the smaller gauge wire with more turns will heat less than a large gauge with fewer turns. Remember the power dumped in the circuit: V2/R, where V is the voltage (12 volts here) and R is the resistance. R is of course proportional to length, and inversely proportional to area. If you reduce the wire diameter x2, you reduce its area x4. That would increase its R x4, if you kept the same length. Since you now have room for 4x as many turns (ignoring the volume of the insulation)  you can increase the length x4. So you end up with R 16x greater, and only 1/16 as much power used.

Depending on the size of the magnet and how it's cooled, you will probably need to keep the power under a few Watts to avoid too much heating in continuous operation. You probably need a wire that's long and thin enough to get R up to some tens of ohms. We're probably talking about tens of meters of thin wire here. There are machines for winding coils, since it's obviously tedious.

Mike W.

(published on 06/17/2009)

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