The possibilities of alternating current by no means stop with using a single source. Ordinary household current in the US is at 60 Hz and about 120 volts. We have, in other words, a single undulating voltage, a smooth sine wave, that completes sixty full cycles of rise and fall each second. But what if another line, also at 60 Hz, were present in which positive and negative crests arrived oppositely to those in the first line? The two sources are, to be exact, 180 degrees out of phase. Because a positive crest (+120v) now corresponds with a negative crest (-120v) in the second line, any device connected across these leads would have 240 volts driving it. Indeed, this is the kind of 240 vac provided to homes for high-power appliances, like electric dryers and hot water heaters.

Though technically there are two phases present in the example above, we refer to it somewhat confusingly as a 240 vac single-phase supply—well, it is a very symmetrical case. In designing some of his ac motors, Tesla employed sources 90 degrees out of phase to fix the stator fields in a desired time relation to one another. For other operating schemes, he adopted three-phase voltages, spaced evenly at 120 degrees separation. This is particularly suitable for heavy equipment, delivering power more smoothly than single-phase circuits.

As it turns out, and as Tesla well understood, three-phase lines are superior, too, for carrying power over a distance. Certain inductive loss features of currents tend to cancel out in a three-phase arrangement. Tesla's unified vision of alternating-current power production, distribution, and use—the world as we know it—is often referred to as his "polyphase system."