Why AC won: the transformer

Two coils of wire on a shared iron ring, not touching. Feed one an alternating current and the other produces a voltage — as large or small as you like, set only by how many times each wire is wound. That device is the main reason the grid runs on AC.

What a transformer is

Two separate coils wound on the same iron core. The primary connects to the supply; the secondary to whatever you want to power. No wire joins them. Energy crosses the gap as a changing magnetic field, and on the way the voltage can be multiplied or divided.

Why it decided a war

Wires waste power as heat, and the waste grows with the square of the current. To send a fixed amount of power a long way you want as little current, so as much voltage, as possible. Nobody wants thousands of volts in a kitchen, so a grid must raise the voltage at the power station and lower it again at the house. With alternating current a transformer does both jobs with no moving parts. With direct current, as we'll see, the same coils give nothing. Changing a DC voltage means first making it change, which takes fast electronic switches — modern power electronics, which the first grids lacked. So AC could be stepped up and down cheaply; DC could not without spinning machines at both ends. The grid was built on AC. (Today, power-electronic converters make high-voltage DC links practical for very long or undersea lines; the everyday grid is still AC.)

How it works

Faraday's law of induction: a coil produces a voltage when the magnetic field threading through it changes. Each turn contributes the same amount, so more turns give proportionally more voltage from the same changing field. A steady field, however strong, produces nothing.

Now put two coils on one iron ring. Iron guides the field around the ring, so the same field threads both coils. Drive the primary with an alternating voltage — swinging back and forth many times a second — and the field swings with it, through every turn of both coils.

Each turn, on either coil, sees the same changing field, so gets the same slice of voltage. The primary's voltage is spread over its turns; the secondary's voltage is that same slice times its number of turns. So the ratio of the voltages equals the ratio of the turns. Ten turns in, a hundred turns out: ten times the voltage.

Nothing is free. In an ideal transformer, power in equals power out, and power is voltage times current. Multiply the voltage by ten and the current must drop by ten. Exactly what a transmission line wants: the same power carried by one tenth of the current, and one hundredth of the heating loss in the wire.

Now try direct current. Switch it on and the field rises, so the secondary briefly gives a voltage. Then the field settles constant, and a constant field induces nothing. The secondary reads zero while the primary passes current through a coil with nothing to oppose it. A transformer on DC is a heater — and a badly overloaded one.

In short

A transformer moves power between coils through a changing magnetic field. Every turn sees the same change, so the voltage ratio is the turns ratio; power is conserved, so raising the voltage lowers the current. A grid can ship power at high voltage with little loss and deliver it at a safe one. Direct current makes a field that stops changing, so the secondary dies — which is why, before power electronics, alternating current won.