If you are staying in a holiday hotel on a hot day in the summer, and your room is at the end of the corridor on the top floor, when you turn on the cold tap it will be several seconds before the water comes out cold.
Similarly it will be several seconds after you turn on the hot tap before the water runs hot.
In the same way, if we switch on a direct current it will take a short time for the equipment at the other end of the wire to reach full power. This does not matter very much if we are just turning on an electric torch, it matters very much indeed on a microchip.
If however we have two tanks, one containing hot water and one containing cold water, and a control valve which allows us to run hot water into a pipe for twenty seconds, and then cold water into the same pipe for twenty seconds, if the pipe is very short after one or two seconds we might get a useful amount of hot water, followed one or two seconds later by a useful amount of cold water, but if the pipe is very long we shall never get any hot water or cold water at all, only water which varies between slightly warm and slightly cool.
Similarly if we pass alternating current along a wire we are constantly reversing the charge on the wire, and if the wire is more than a certain length nothing useful will reach the other end. For reasons not explained here this certain length depends upon what the wire is made of and, just as importantly, how it is insulated and protected. The best material to use is nothing at all (a vacuum), but air is almost as good: an overhead cable needs to be more than 100 km long before the degradation is significant. But if it is covered with the sort of material needed for an underwater cable it cannot be more than a few hundred metres long. This is why if electricity cables need to cross a river they are always run over it or through a tunnel under it.
We can overcome this length restriction by using high voltage direct current: we convert the AC to DC at one end and then the DC to AC at the other end. Originally the technology to do this simply did not exist (other than by using AC to power an AC motor to drive a DC generator at one end and then using the DC to power a DC motor to drive an AC generator at the other, in effect a power station at each end of the cable). Various experimental methods have been tried, but since the 1970s modern semi-conductor technology has made it a totally practical reality.
However the equipment needed to handle gigawatts of power is at present very expensive and needs a lot of space at each end so it is not usually used for overhead cables on land except for cables that run uninterrupted for at least a thousand kilometres, for example those carrying electricity from the Rocky Mountain hydro-electric power stations in the northern United States to power all the power-hungry air-conditioning systems in Arizona and Nevada. But it is essential for undersea power cables: the longest so far is 590 km and runs under the North Sea from Norway to the Netherlands.
