How to Make a Very Large Electromagnet
By Jesse Randall
Electromagnets perform an act which at first appears to be purely magic. By applying an electric current through a coil of wire wrapped around an iron shaft, the entire ensemble immediately turns into a deceptively strong magnet, complete with a north and south pole. By increasing the current traveling through these windings, the power of the magnet can be directly manipulated. With sufficient power, electromagnets become stronger than any natural magnet on the planet and can be used commercially from lifting entire cars off the ground to scanning the inside of a patient's brain in an MRI.
Design the electromagnet. The current capacity of 20 AWG wire is about 3 amps, so by dividing 12 volts (the battery supply) by 3 amps, the yield is 4 ohms. The finished coil must be 4 ohms total. Since 20 AWG wire has a resistance of 0.0119 ohms per foot, this means that a total of 336 feet of wire is needed to produce the electromagnet.
Determine some size parameters. The 1/2-inch bolt, arbitrarily chosen, has a circumference of about 0.131 feet. Dividing 336 feet of wire by 0.131 feet of the bolt circumference gives 2,566 wraps. Since 30AWG wire has a thickness of about 32/1000 of an inch, the coil will be about 82 inches, but since the bolt is only 6 inches long, this will only wrap to a thickness of about 1.376 inches total. Therefore, 2-inch fender washers are sufficient to provide the edges of the coil form.
Slip the 2-inch fender washer over the bare bolt. Then slide on the metal tube and the other fender washer followed right after. Screw on the nut and tighten with a wrench.
Wind the 30 AWG coil around the smooth shaft of the steel tubed bolt. Begin by leaving a tail of the wire of about two feet and taping it to secure it while starting the initial winding. Hand wrap the 2,000 or so wraps in a uniform and systematic matter going from one end of the bolt to the other and then back. Continue until 336 feet of wire are installed. Cut the wire and leave the end tail about two feet long as well. Secure in place with electrical tape.
Remove about an inch of the insulation enamel from the copper wire on both ends by lightly scratching with a knife.
Connect the two ends to the two terminals of the car battery. The 4 ohm load will draw 3 amps from the car battery, and since the car battery is a 12 volt supply, a total of 36 watts will be sent to the load. Much of this will be lost as heat, but the remainder will be converted to magnetism. The actual efficiency is difficult to calculate, but can be tested by measuring the maximum weight the electromagnet can lift and from what distance its influence affects.
- When connecting to a car battery, wear gloves and safety goggles. A 3 amp load applied to a car battery, if improperly designed (if it can't handle the heat) it can immediately catch on fire. Be careful.
Jesse Randall studied mathematics and physics and works as an embedded electronics engineer, developing microcontroller firmware and digital interfaces. He writes about subjects including abiogenesis, electrochemistry and algorithm optimization. He has been writing on technology-related subjects since 2000.