Explanation of electric nomenclature can best be given by the analogy in hydrostatics of a stream of water passing in the hose pipe from a fire-engine. The “watt” indicates the sum total unit of electrical power for a definite period of time, and in the hose pipe would be represented by the effective force of a definite volume of water, passing at a definite pressure, during a definite period of time. “Volt” is a pressure unit of electro-motive force, and would be represented by the power of the engine. “Ampere” would be the quantity, or volume unit, or cross section of the hose pipe, and the “ohm” would be the unit of frictional resistance. The “watt” then would be the “volt” multiplied by the “ampere”; thus 500 watts would be 10 amperes at 50 volts, or 50 amperes at 10 volts. Low tension circuits, such as are used for incandescent lights, range from 100 to 240 volts and are harmless. Trolley circuits are usually 500 volts, and will kill an animal, but are not necessarily fatal to man. High tension currents from 2,000 to 5,000 volts, such as are used for arc lights, are fatal. Of all modern inventions, not one has advertised itself in such a spectacular way as the electric light. Those who have seen the magnificent electrical displays at the Chicago Fair, the electrical celebrations in New York, and the Omaha Exhibition, need no introduction to its marvelous splendors and beauties. In the annual report for 1898 of the Edison Electric Illuminating Company of New York, its statement shows that for that city alone the gross earnings were $2,898,021. There were 9,990 users of the electric light, 443,074 incandescent lamps, and 7,353 arc lights. It is estimated that the electric light stations and plants in the United States alone amount to $600,000,000. In the year 1899 a single manufacturing concern (The General Electric Company) received orders for 10,000,000 incandescent lamps, which is about one-half of the present annual production. Sixteen years ago the lamps were $1 each; to-day they can be bought for 18 cents. What the future has in store for the further development of the electric light no one may dare predict. Already a different form or manifestation of electric light has been demonstrated, in which neither the electric arc nor the incandescent filament is used, but a peculiar glow is seen disassociated from a direct material habitation, and produced by currents of enormous frequency and high potential, in accordance with the patent to Tesla, No. 454,622, June 23, 1891. Other worthy inventors in this field are at work, and its development will be one of the interesting problems of the Twentieth Century. CHAPTER VIII. THE TELEPHONE.