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Why alternating current?

Late one evening, when it is getting quite dark, you need to get something from the garage of your house. There is no power point or electric light in the garage.

We measure the amount of light given out by a lamp or candle in lumens. A well-lit room needs about 1800 lumens but to find what you are looking for in the garage you only need about 400 lumens. For the record the direct light of the Sun is about 100 000 lumens, which is why you must never ever look straight at it.

There are four ways you can obtain 400 lumens, all using low-energy LEDs.

We measure power in watts. All LEDs have about the same efficiency in converting electrical energy into light, about 80 lumens per watt, so all four sources use about 5 W. (An old-style incandescent lamp, one with a filament which gives out light only because it is heated until it is white-hot, is much less efficient and gives out only about 15 lumens per watt: almost all of the electrical energy is converted into heat not light.)

The power used by an electrical appliance depends upon both the voltage and the current.

Volts × Amps = Watts


All the lamps give out the same amount of light and use the same amount of power but they are not interchangeable: the resistance of a lamp (or anything else) is found by dividing the voltage by the current, so the resistance of the 3 V lamp is about 2 ohms, the 9 V lamp about 22 ohms, the 12 V lamp about 40 ohms and the 230 V lamp about 12 000 ohms. If you connected the 230 V lamp to a 3 V battery it would not work at all, if you connected the 3 V lamp to the 230 V mains it would be destroyed.

What this means is that we can get the same amount of power by using a higher voltage and a lower current or a lower voltage and a higher current.

If we connect a 1 kW heater to a 230 V supply the current will be just under 5 A so we must use a cable capable of carrying 5 A without getting too hot. No problem: you can buy a few metres of a suitable cable for a few pounds. But if we are thinking about a whole town, not just one heater, we are thinking of hundreds of kilowatts and thousands of metres, and the problem is that because the heat produced by an electric current flowing along a cable depends upon its resistance and the square of the current (I² × R) a cable designed to carry ten times the current needs a hundred as much copper. We can greatly reduce the current, and so the amount of copper needed, by transmitting the electricity at thousands rather than hundreds of volts. For example, 10 000 W at 100 V needs a current of 100 A but at 10 000 V the current is only 1 A. It would however be very dangerous to run a cable carrying 10 000 V into an ordinary house. What we need is a method of reducing the voltage to a safe level before it comes into the house. Using alternating current this has always been very simple, we just need a transformer, but with direct current it is, or at any rate was, very difficult indeed.

The great pioneers of mains electricity were two Americans, Thomas Edison (1847 - 1931) and George Westinghouse (1846 - 1914), although the technology quickly spread to Europe and the rest of the World.

In the late 1880s “The Battle of the Currents” was fought in the United States of America between Edison, who had invented the incandescent electric light bulb in the 1870s and was busy setting up a 100 V direct current system to create a market for it, and Westinghouse, who was developing a 240 V alternating current system, based upon work done by Nikola Tesla (1856 - 1943). Westinghouse’s alternating current system won largely because of the construction in 1895 of a hydro-electric plant at Niagara Falls: it simply was not feasible to transmit the huge amounts of power from it to the towns of the United States and Canada as anything other than high voltage alternating current.

Today alternating current is used throughout the world although high voltage direct current is used for underwater cables and this is discussed on its own Page.

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© Barry Gray October 2017