Page:Popular Science Monthly Volume 45.djvu/647

Rh high voltage on account of the very much more perfect insulation possible.

The advantage, of course, of using a very high electrical pressure lies in the principle stated above of the loss in sending a given amount of energy over a given wire being inversely proportional to the square of the voltage.

By the use of step-up transformers it will, of course, be possible to transmit at any voltage that the insulation of the line can withstand; but if this high voltage could be reached by the machines directly, the loss (we may liken it to a friction loss in machinery) of efficiency in the transformers, and, even more important, the great cost of that part of the equipment, would both be avoided.

What will be done will be to use these step-up transformers and put current on the transmitting line at about twenty thousand volts; it is likely, however, that in any subsequent enlargements of the generating plant the three original machines will be used for local work only, and a radical change made in the direction of an enormously higher generated voltage.

Intimately associated with this question is the problem of how to convey current at this tremendous potential of twenty thousand volts to distances. An idea of what it means may be had from the facts that two thousand is relied on to be sufficient to instantly kill a human being, and that the energy of a current given up in passing through any given resistance varies as the square of the voltage.

The chief difficulty to be met in such line construction is that of efficiently insulating the wires. If one attempted to use a line insulated merely as an ordinary telegraph line is, there would be an enormous loss, amounting practically to the whole of the transmitted current, in moist weather, by leakage over the damp surfaces of the glass or other insulators. The remedy for this leakage would, however, be a comparatively simple matter by means of well-known oil-holding arrangements for the insulators were it not for the further fact that it is imperatively necessary not to have the two wires, the going and return ones, farther apart than can not be avoided on account of what are known as the effects of self-induction. The wires strung on telegraph poles would have to be so far apart in order to insure their never, by any possibility, coming in contact, that the self-induction losses would make that method impracticable.

The self-induction of a circuit has the effect of retarding both the starting up and the dying out of a current flowing in the circuit. The phenomenon gives a resemblance of the possession of a property analogous to mechanical inertia to the current. Since inertia, however, is a property dependent solely on the mass of a