An elementary book on electricity and magnetism and their applications . thetwo coils respectively thevoltage coil and the currentcoil. The action of such awattmeter is best explainedby an illustration (). Suppose it is desiredto measure the power usedby an electric motor. Thecurrent coil of the watt-meter is connected in serieswith the motor, and thevoltage coil is connectedacross the terminals of themotor. The magnetic ef-fect of the current coil isthen proportional to the current which flows through the motor,and the magnetic effect of the voltage coil is proportional tothe voltage a
An elementary book on electricity and magnetism and their applications . thetwo coils respectively thevoltage coil and the currentcoil. The action of such awattmeter is best explainedby an illustration (). Suppose it is desiredto measure the power usedby an electric motor. Thecurrent coil of the watt-meter is connected in serieswith the motor, and thevoltage coil is connectedacross the terminals of themotor. The magnetic ef-fect of the current coil isthen proportional to the current which flows through the motor,and the magnetic effect of the voltage coil is proportional tothe voltage at which the current is supplied to the indications of an electrodynamometer are proportional tothe product of the magnetic effects of the two coils. Conse-quently, in this case the indications are proportional to currenttimes voltage, or watts, instead of current times current (,current squared) as in the Siemens electrodynamometer. Itshould also be remembered that a wattmeter, unless compen-sated, indicates the power consumed by the load plus the power. Fig. 110. Measurement of power bywattmeter. 176 ELECTRICITY AXD MAGNETISM Shunt Circuit Counting Geai consumed by one of its coils. When the power to be used issmall, this error is appreciable and must be corrected. These direct-reading wattmeters can be used for alternatingas well as direct currents and may be made in a portable formto test the efficiency of generators, motors, lamps, or any otherelectrical device. The accuracy of a wattmeter may be testedby comparing its readings when connected with a d-c. circuitwith the indications of standard voltmeters and ammeters. 126. Watt-hour meter. Watt-hour meters are used to showthe amount of energy used each month by the customers of electric plants. A common form ofmeter used for this purpose is thatshown in figure 111, known as theThomson watt-hour meter, or inte-grating meter, after its inventor,Elihu Thomson. This is built likea little electric motor but without
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