534 A MANUAL OF PHYSIOLOGY 



on each other, and that this energy is transformed into heat (p. 72). 

 In like manner electrical energy is transformed into heat whenever a 

 current flows along a wire. The heat produced in a circuit in which 

 no external work is done is exactly equal to the energy which has dis- 

 appeared in the transference of the electricity from the place of higher 

 to the place of lower potential ; just as the heat produced in the flow 

 of a liquid is equal to the difference in its total energy at the 

 beginning and end of the path. If C is the current strength, and E 

 the electromotive force, the energy represented by the transference of 

 electricity in time / is EC/, or (since E = CR by Ohm's Law), C 2 R/; 

 and this represents the heat produced in the circuit when no work 

 is done. 



For the measurement of electrical quantities a system of units is 

 necessary. The common unit of resistance is the ohm, of current the 

 ampere, of electromotive force the volt. The electromotive force of a 

 Daniell's cell is about a volt. An electromotive force of a volt, 

 acting through a resistance of an ohm, yields a current of one 

 ampere ; but the current produced by a Daniell's cell, with its poles 

 connected by a wire of i ohm resistance, would be less than an 

 ampere, because the internal resistance of the cell itself, that is, the 

 resistance of the liquids between the zinc and the copper, must be 

 added to the external resistance in order to get the total resistance, 

 which is the quantity represented by R in Ohm's Law. 



Measurement of Kesistance. To find the resistance of a con- 

 ductor, we compare it with known resistances, as a grocer finds the 



FIG. 147. WHEATSTONE'S FIG. 148. DIAGRAM OF RESISTANCE 



BRIDGE. Box. 



weight of a packet of tea by comparing it with known weights. The 

 Wheatstone's bridge method of measuring resistance depends on the 

 fact that if four resistances, AB, AD, BC, CD, are connected, as in 

 Fig. 147, with each other, and with a galvanometer G and a battery F, 



AB BC 

 no current will flow through the galvanometer when - = p^pr. 



A.LJ \^\-f 



For when no current passes through the galvanometer, B and D 

 are at the same potential. Let the fall of potential from C to B or 

 from C to I) be a ; then, since the total fall of potential from C to A 

 must be the same along either of the paths CBA or CDA, the fall from 

 B to A must be equal to that from D to A. Call this /?. Now, the 



