Large dynamos generate electricity very much more cheaply than small machines can, and machines which have a full load continually produce the current very much more cheaply than those which run upon very light load part of the time. The largest central stations with load evenly distributed for the whole day could furnish electricity profitably at four cents per kilowatt hour. There are many small electric lighting plants which furnish current from sundown to midnight only at fifteen cents per kilowatt hour, with little profit. The transformer ([Fig. 127]) makes it possible to gather all this generation of electricity for sparsely settled districts into large central stations, located sometimes far away from the consumer perhaps, where there is abundant power in some water-fall, thus saving the expense of coal for running the dynamos.
Photograph by Helen W. Cooke
Operating the Switchboard
A few years ago there were no central stations for this purpose. Now according to the latest census reports there are in the United States about 30,000 plants, including those which belong to certain cities, that generate electricity for sale, and there are twice as many more isolated plants to furnish light and power in factories, hotels, etc.
Fig. 127
The money invested in central station business now exceeds six billion dollars, and the annual output of electric current is sufficient to keep eight billion 16-candle-power carbon filament electric lights burning continuously night and day. All this has more than doubled in the last five years. Central stations are now furnishing about five times as much current for heating, cooking, and charging automobiles as they did five years ago. About one third of all the central stations depend on water power.