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Down through the centuries in which electricity remained a natural mystery, and later a fashionable novelty, it turned up only in the form we would term today direct current (DC), that is, with electrons moving in one direction only. The first, cumbersome batteries (called voltaic piles) and mechanical curiosities that built up static charge (like Leyden jars) provide electrons that stream in one direction. Even the famous experiments of Benjamin Franklin utilized a direct current supply—lightning.

There's certainly nothing inferior about a direct current, unless you are trying to solve practical engineering problems concerned with generating power and distributing it over great distances. A few visionaries, Tesla foremost among them, comprehended both that the new science of electricity must be, literally, transformed and that the means already existed in theory—as well as in some wheezy devices usually found in physics labs of that era. The solution lay in alternating currents (AC).

What Is Alternating Current?
An AC source produces currents that flow in one direction and then the other, continuously cycling through peak values in either direction, i.e., first positive, then negative, and so on. The advantages—which turn out to be nothing short of revolutionary—are not immediately obvious; they derive chiefly from that magnetic property of currents, induction.

Direct currents don't cause much inductive action. When a switch is thrown and current first flows in a DC circuit, a magnetic field builds up. The field can induce a current to flow in any nearby wire, but only briefly, just during the few instants it takes for the current to get moving. In fact, Michael Faraday was led to his discoveries in induction by first noticing the momentary currents induced by a DC source he had turned on. Once the field is built up, induction stops; the field's force lines are stationary and no longer carrying a change of energy through space and cutting across nearby wires.

With an alternating current the magnetic state of affairs is never a settled one. Each time current direction reverses, so must the pole orientation of its associated magnetic field. The entire field collapses and rebuilds in the magnetically opposite direction. If current alternates continuously, the field is never static. Alternating currents do, in a sense, copy their changes of energy into nearby circuits, making energy available there. Though all very clever, it may seem this isn't a prize winning trick; why not just connect the two circuits with a piece of wire? Why complicate matters with induction?

Transforming AC
It's not just a question of getting power to a nearby circuit; induction can be made to change the form in which power is delivered, it can be transformed, in the electrical sense. Manipulating the way fields are concentrated—usually by making coils of the conductor—will change the properties of currents and voltages that a source (the primary) induces in another, nearby set of coils (the secondary). For example, power present in the primary as a large current at a low voltage may be transformed into low current at high voltage in the secondary.

AC Advantages
Generally, engineers would much prefer to send power over long lines at a very high voltage, with comparatively lower current, but deliver it to most users at a safer, lower voltage. Transformers make that possible. Resistance in AC circuits works differently, too, so that with good design, losses in power lines are dramatically lower than in DC lines. (The first DC power stations could only serve an area within a few mile radius.)

The same basic AC ideas, a magnetic transfer and transformation of power, can also make highly efficient, reliable motors. One obvious advantage, though there are many, is the spinning part, the rotor, need not be connected physically to any electrical contacts; ever changing fields in the stator (stationary part) convey the power. Nor are AC devices limited to a single AC source; several may be supplied simultaneously in a polyphase arrangement.

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Inside the Lab Index
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Learn more about the qualities and behaviors of electricity:

Electrons and Current

Magnetic Fields

Particles and Waves

Voltage

Power Transmission

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