Hi Leslie- It seems that you've already read some of our old answers about making magnets. So you know that you want to get as big a field as possible on the nail, and that the best way to do that is with pulses of current through your coil. Pulses let you get brief big fields without getting the coil so hot as to damage it.
As for the car battery, it sounds perfect. Its voltage is low enough to be safe, and it can supply a lot of current. Then when it starts to run down, you can recharge it. But be careful- it can store enough energy to fry your coil, so be sure not to leave the current on for more than a brief pulse.
You might want to design your coil to match your battery. The battery should have some rated current (say 100 A, for example) which it can supply before its voltage starts to drop much below 12 V. You would want the resistance of your coil to then be about
12 V/100 A = 0.12 ohm so that it makes full use of your battery’s ability to deliver power. Remeber again, that power (in our example) would be about 1200 W, as much as a hair dryer, so your coil will heat up very quickly and cannot be left on. Use a contact switch that goes off as soon as you take your finger off, and for extra precaution keep the coil away from anything flammable- say on a nice slab of concrete or rock.
So you want to pick a copper wire gauge so that a one-inch long coil, wound with enough layers to have about a half-inch in diameter, has a resistance of around 0.12 ohms.
Good luck, and please don’t start a fire.
(As you can see, there are many lawyers in America.)
(published on 10/22/2007)