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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1911 Excerpt:...equation (320), and the difference of potential measured over this circuit will be the same as over AB, and equal to the original difference of potential, unless the current through the original circuit is appreciably changed by the introduction of the new resistance. If ADCB form the coil of a galvanometer the current through this instrument produces a deflection. Evidently the galvanometer may be calibrated in such a manner that the readings give directly the difference of potential existing at its terminals. The readings of such an instrument therefore indicate, not the current, but the product of the current flowing through it into its own resistance. The general form of the voltmeter (Fig. 142) is the same as that of the ammeter. 269. Electric Energy, Electric Power. Electric energy is energy measured in terms of electric quantities. We have seen (Art. 264) that it may be expressed as Electric Energy = I2Rt-or, introducing the difference of potential E, Electric Energy = Elt = EQ (321) Remembering that an ampere is 10-1 c. G. s. unit, an ohm 109 c. G. s. units and a volt 108 c. G. s. units, electrical energy is given in joules or 107 ergs, if the electric quantities be measured in practical units. A volt coulomb is therefore identical with one joule. Electric power is the time rate of expenditure of electrical energy, or Electric Power = EI (322) One volt ampere is identical with one watt. A kilowatt is 1000 watts. In commerce, the unit of energy used is the watt hour = 3600 joules, or the larger unit, the kilowatt hour = 1000 watt hours. If it be desired to measure the heat in calories, produced by the absorption of electrical energy, we have, since one joule equals 0.24 calorie (Art. 177), E = IBt joules = 0.24 IRt calories (323) where all electric q...