ELECTRICAL MEASUREMENTS. When electric motive force of one volt passes through a resistance of one ohm it produces one ampere, hence an ampere is a combination of two things, electric motor force and resistance. An ampere will do a certain amount of work, will decompose a given weight of water, and will deposit .005084 grain of copper per second and in several ways it may be demonstrated what an ampere is. An ampere measurer is just as necessary to all constant current circuits as is a steam guage to a steam plant. Some ammeters are designed to be used only for standards of comparison and others are intended and so constructed that they can remain continuously in circuit. Each instrument, if well constructed, has considerable range. See Fig. 47. ELECTRICAL MEASUREMENTS. The Voltmeter is shown in Fig, 48. It is different from the ammeter in having high resistance and is connected between the two poles or positive and negative wires. The voltmeter measures electric pressure just as the steam guage measures the pressure of steam. In the illustration two scales are indicating volts and the inner scale shown, the outside one one-twentieth of a volt. An instrument that will measure accurately from 1 to 100 volts will not give good results for measuring potentials of 1,000 to 5,000 volts. An ammeter that will measure accur ately the thousandths of amperes in the current of an induo ELECTRICAL MEASUREMENTS. tion coil, will not do at all for measuring 100 or 1,000 amperes. Fig. 47 shows the standard ammeter and Fig. 49 instrument for use to indicate large station output of electricity. The Station Meter.-Electric energy is now supplied in large quantities and distributed for lighting, motive power and heating from large central stations or power-houses. From the power-house distributing mains of copper go out consisting of "feeders" leading into the network of conductors. The station meter, illustrated in Fig. 49, is designed to indicate with extreme accuracy the large units being constantly produced and utilized. The Watt Meter.-The watt is an electric unit obtained by multiplying the volt, which represents pressure, into the ampere, which represents volume; hence, a meter designed to measure electricity sold to customers for motors, lamps and heaters is called a watt-meter or measurer and is illustrated in Fig. 50. The watt-meter consists of two coils of insulated wire, one fine and one coarse, so arranged in the electric circuit that they act upon each other. These produce a motion in a train of clockwork by which the electric energy is recorded on dials in the same manner as gas consumption is recorded in a gas meter. This record is made in watt hours, being I volt X I ampere X I hour-one watt hour. |