"Hydro-electric stations are the proper thing now," I remarked. "On the Rio Grande River in Colorado they are constructing several plants where water power will be utilized to generate electricity for use more than one hundred and fifty miles away. For transmitting electricity to such a distance they step up the voltage, or electro-motive force as it is called, to 100,000 volts. They are harnessing the Au Sable River in Michigan to generate electricity and transmit it at 135,000 volts e. m. f. to towns nearly two hundred miles away. Electricians use e. m. f. for electro-motive force, just as you boys use "exams." as slang for the motive force in school. Of course we are aware that since 1896 some of the water power of Niagara had been converted into electric power to run street cars and factories and furnish electric light and electric heat as far away as Buffalo, twenty-six miles distant. About $18,000,000 are now being invested in hydro-electric enterprises even in Mexico. By this time the boys were all standing up and staring at me, while Harold inquired if I were talking in my sleep. "I have at any rate succeeded in waking you all up," said I, "and what I have said is not altogether a joke. Let me explain somewhat at length." XII ELECTRICITY FROM CENTRAL STATIONS 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. [Illustration: 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. [Illustration: Fig. 127]