. Electric railways, theoretically and practically treated . rst point and note if any deflection occurs. If a deflection does occur, adjust resistance in R, untilzero deflection takes place. Then throw galvanometerswitch to the second point, and adjust small resistancesin R until no deflection occurs. The circuit is then bal-anced and the total resistance included in the external cir-cuit, that is, between B + and B — , divided by 1,000 ohmswill yield the true voltage of the external circuit. Thisvoltage should be compared with the voltmeter readingwhen the circuit is balanced. As the resista
. Electric railways, theoretically and practically treated . rst point and note if any deflection occurs. If a deflection does occur, adjust resistance in R, untilzero deflection takes place. Then throw galvanometerswitch to the second point, and adjust small resistancesin R until no deflection occurs. The circuit is then bal-anced and the total resistance included in the external cir-cuit, that is, between B + and B — , divided by 1,000 ohmswill yield the true voltage of the external circuit. Thisvoltage should be compared with the voltmeter readingwhen the circuit is balanced. As the resistance of this box is limited to approximately100,000 ohms, it is obvious that some other ratio than 274 ELECTRIC RAILWAYS. i,ooo ohms per volt must be chosen to make high voltagecalibrations and also extremely low voltage calibrations. Calibration of an Ammeter. — In commercial practice itis preferable to carefully calibrate a standard voltmeter,and to check ammeters against it through the medium ofa standard ohm. The ammeter is placed in series with the. Fig. 171. — RAILWAY TYPE RECORDING WATTMETER. Standard ohm, and an adjustable source of , the volt-meter shunting the standard ohm. It is obvious that thevoltmeter and ammeter readings should correspond foreach set of readings when both instruments are in perfectcondition, as RR = I ohm, therefore, I — E. ■ ELECTRICAL MEASUREMENTS. 275 Thompson Recording Wattmeter. — This wattmeterconsists of a delicate compound wound motor whose arma-ture shaft is connected to a train of wheels registering itsmotion upon a dial similar to that of a gas meter. Thearmature of the motor in series with a high resistance isconnected across the service wires. As the resistance of thearmature and the series resistance is constant, the currentpassing through the armature will be proportional to the r^n
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