. The principles of physics. ELECTRO-MOTIVE POECB OF DIFFERENT CELLS. 507 in which r is the battery resistance, R is the resistance in the re-sistance box; a and 6 are the resistances respectively in the armsA 6and AD. Ha = 6, the for-mula is siipplifled, and becomesj- = R. The information generally re-quired in practice, however, iswhat available difference of po-tentials can be obtained with acertain working resistance in theexternal circuit. This can beobtained by connecting the ter-minals of the battery by thebody offering this resistance, andmeasuring the difference of po-tentials between
. The principles of physics. ELECTRO-MOTIVE POECB OF DIFFERENT CELLS. 507 in which r is the battery resistance, R is the resistance in the re-sistance box; a and 6 are the resistances respectively in the armsA 6and AD. Ha = 6, the for-mula is siipplifled, and becomesj- = R. The information generally re-quired in practice, however, iswhat available difference of po-tentials can be obtained with acertain working resistance in theexternal circuit. This can beobtained by connecting the ter-minals of the battery by thebody offering this resistance, andmeasuring the difference of po-tentials between these points bymeans of a potential galvanom-eter. If we call this differenceof potentials V, and the the battery when on open circuit E, and R = external resistance,then we may write R + r R where r is such a quantity as satisfies this equation. In otherwords, this quantity may be taken as the resistance of the batteryfor the current Fig. 394. Section XI. OF DIPFEBENT CELLS. DIVIDED COMBINING VOLTAIC CELLS. METHODS 476. Electro-motive force of different cells. — If a galva-nometer be introduced into a circuit with different battery cells, Bunsen, Daniell, Grenet, etc., very different deflectionswill be obtained, showing that the different cells yield cur-rents of different strengths. This may be in some measuredue to a difference in their internal resistance, but it is chieflydue to the difference in their electro-motive forces. We 508 ETHER DYNAMICS. learned (§ 431) that difference of electro-motive force isdue to the difference of the chemical action on the two platesused, and this depends upon the nature of tjje substancesused. It is -wholly independent of the size of the plates ;hence the electro-motiye force of a large battery cell is nogreater than that of a small one of the same kind. Conse-quently any difference in strength of current yielded bybattery cells of the same kind, but
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