Minimum-resistance coil design

Significant reduction of coil resistance is possible through use of a divided-winding technique. Since the two portions of the winding have equal resistance and the same number of turns, they may be connected in parallel as well as in series One of the most frequent problems in electrical engineering is the design of a coil to produce a magnetic field within a pole. The device may be a relay, a field coil, a holding magnet, a transformer, or an inductor. In general, the flux through the pole is the desired result; the power dissipated in the resistance of the coil is the price that must be paid. For some applications, such as relays, high-frequency transformers and ferrite-core inductors, efficient design leads to a coil with an outer diameter large compared to the pole diameter. In such cases it is possible to obtain a lower resistance per given number of turns by constructing the coil from two or more sizes of wire, with the smaller sizes on the inside, than by winding it with wire of one size. The improvement is generally not large but is often significant; there are substantial fringe benefits, and the added cost and complication are slight.