Voltage and current

Electric power is generated at power stations, but it is usually needed for far-off places. How is the current taken to these far-off places?

Thick wires usually carry it across the country, and steel pylons hold the wires above the ground. The pylons are so high that nobody can touch the wires at the top. The wires are not usually copper wires; they are made of aluminium, and thirty wires together form one thick cable. Aluminium is so light that the pylons can easily hold the cables up.

It would not be cheap to drive very large currents through these cables. Large currents need very thick wires. If thin wires are used, they get hot or melt, and so the currents ought to be as small as possible. Can we send a lot of power if we use a small current? We can do so if the voltage is high. We need a small current and a high voltage; or a large current with a low voltage. The small current is cheaper because the wires need not be thick.

The result is that the voltage has to be very high. The pressure in the aluminium cables may be 132,000 volts, and this is terribly high. The voltage of a small battery which we carry in our pockets is usually between 1 and 9 volts. A car battery has a voltage of 6 or 12 volts. In a house the pressure in the wires may be 230 volts, or something like that.² Even 230 volts is high enough to kill a person, so what would happen if we touched one of the aluminium cables? The high voltage would drive a heavy current through our bodies to the earth.

When the wires lead down to a house or a railway, the voltage is made lower. It can be changed easily; but if the voltage is lower, the current must be higher. If it is not, we shall lose power. So the wires have to be thicker.

The wires must never touch steel pylons. If they did that, the current would escape to the earth through steel. Steel is a good conductor of electricity, so are most metals. We have to separate the wires from the pylon, and we do this with insulators.