Page:Popular Science Monthly Volume 45.djvu/641

Rh details in the design and proposed method of utilizing the current of the generators, we may glance at what has been decided on, and review the more important points raised in connection therewith.

In the first place, the use of an alternating as opposed to a direct current was decided on, as was to have been expected. The development within the last year or two of alternating-current motors has rendered possible the distribution of electricity for power (as opposed to lighting) purposes over distances before almost out of the question. It has been for a number of years past possible to transmit large quantities of electrical energy for lighting which was not suitable for running the then known motors. The method of electrical distribution for lighting purposes that is used in cities is available also for transmission to considerable distances. It consists, as is well known, of a dynamo supplying current at a high voltage to the street lines, and a system of transformers each taking a portion of this current at high voltage and giving in return a current of greater amperage or volume and of lower voltage for house consumption, the object being simply to avoid loss of voltage or pressure by transmitting a heavy current over a light wire. As this may not be quite clear to every reader, it may be as well to say a little more about it.

The energy of any current is determined by and is equal to the product of two of its properties, its volume or ampèrage and its pressure or voltage. Letting C represent the ampères and V the voltage, we have that the energy $$=$$ C V. In passing any current over any wire there is a loss of voltage determined by and equal to the product of two things—i. e., the ampèrage of the current and the resistance of the wire; so we have loss of voltage $$=$$ C R. Now, if we have two currents—one, say, of ten ampères and one volt, and the other of one ampère and ten volts—the energy will be the same, or ten watts as it is called. If we pass both through a given resistance, R, we shall have a loss of voltage ($$=$$ CR) ten times greater in the first than in the second case. But a given loss of voltage amounts to only one tenth as much energy (C V) in the second case with C $$=$$ one ampère as it does in the first with C $$=$$ ten ampères, so that with only one tenth the given loss of voltage the energy lost will be only one one-hundredth that lost in the first case. What it amounts to is that the loss in passing a given amount of electrical energy through a given resistance is proportional to the square of the current, or amperage, and consequently inversely proportional to the square of the pressure, or voltage.

If, therefore, current is used in a house at fifty volts and transmitted to the house at one thousand volts, the loss will be