Building an Electromagnet - Some Practical Rules-of-Thumb

For a given thickness of wire wound on a nail, more wraps of wire will give more magnetizing of the nail. As the amount of wire used gets longer, the electrical resistance grows, but (because the total resistance also includes that of the battery) the resistance does not grow by so great a proportion as the number of wraps does, so the benefit of additional wraps tapers off, but does not go to zero.

Since what counts is the number of wraps, not the wire length itself, it helps to wrap the wire efficiently so as to give the greatest number of turns for the length — wrapping the turns tightly, smoothly, close together, without zigzags and crossings that take extra length.

Also since the magnetic field of any given turn of wire will spread out with distance along the nail, to get the most concentrated total field at one end of the nail (to pick things up) it will help to wrap all the turns near that end of the nail. There will then be a trade-off to explore, about how good it is to stack up multiple layers of windings near the nail's end — which puts more turns near the end but means that the outer layers cost extra wire length and are not so close to the iron of the nail.

Thicker wire allows more current but also takes more room, so that the number of turns that will fit in a given space will be less. (Magnets designed to be used inside manufactured products often have a limit on the space they can take.) If the winding space available (or the total weight of wire allowed) is fixed, so that fatter wire means shorter windings, there will be an optimal wire thickness beyond which performance drops off (and batteries get drained fast!) because the windings’ electrical resistance gets so low that the battery’s own resistance drains the battery’s power more than the wire. In these conditions, the battery consumes itself.

This optimum comes when the winding resistance is equal to the battery’s own internal resistance. The battery resistance itself cannot be measured directly with an ohmmeter since the battery’s voltage gets in the way. Yet the external load whose resistance matches the battery's internal resistance can be identified with a voltmeter -- it will be the one that results in the battery voltage dropping by 50 percent while the load is connected. [NOTE: a drain this large should not be imposed on the battery for very long!] This matching of resistances is a simple example of a general principle called impedance matching, a broadly applicable strategy that results in maximizing the power delivered from a source to a task.