1893.] 



the Mechanical Equivalent of Heat, §c. 



11 



standard are small, and their mean E.M.F. at 15° C. = 1*4344 



Volts.* 



R'. — Had it been possible to maintain a constant value for R, it 

 would have greatly simplified both the experimental work and the 

 calculations. In the year 1890 we devoted much time to the exami- 

 nation of the various copper-manganese-nickel alloys, and we per- 

 formed a series of determinations with a coil whose temperature 

 coefficient was practically zero. The reasons which led us to reject 

 these alloys and adopt a platinum wire will, we think, be found 

 sufficient. 



The value of R was first determined by a dial-box (legal ohms) 

 constructed by Messrs. Elliott.f Mr. Glazebrook has been so kind as 

 to perform a complete standardisation of this box by means of the 

 B.A. standards. The resulting corrections have been applied, and the 

 values of R expressed in true ohms, as defined by the ' B.A. Report,' 

 1892. 



If R is the resistance of the coilj when at the standard tempera- 

 ture 0, then R r = R{1 + k(0i + (3— 0)}, where k is the temperature 

 coefficient of the wire and ft is the excess of its temperature above 0, 

 the temperature of the calorimeter. It is difficult to describe in a 

 few sentences the manner in which we determined the value of /3, 

 but the following explanation may serve to indicate the method of 

 procedure. 



Suppose P, Q, R, and S to be the arms of a Wheatstone's bridge of 

 which S is the coil. Let the arms P and Q be equal, not only in 

 resistance but in mass and dimensions, and let R = S when the coil 

 is at a certain temperature 9 S , the reading of the thermometer in the 

 calorimeter being lm Let R be built up of a large mass of meta 1 

 having a small temperature coefficient and a considerable cooling 

 surface. § If the bridge is balanced when a certain current is passed 

 through it, the balance will be destroyed if the temperature of the 

 coil S be raised as the current is increased, for the increase in tem- 

 perature of R may be neglected, and P and Q will remain equal, 

 however their values alter, since they are traversed by equal currents 

 and their capacities for heat are the same. Equilibrium can, how- 

 ever, be restored by cooling the calorimeter to a certain temperature, 



* A full description of these cells will be found in Messrs. Glazebrook and 

 Skinner's paper ('Phil. Trans.,' 1892, pp. 622—624). 



f Particulars of this box have been given in a previous paper (' Phil. Trans./ A, 

 1891, p. 44). 



t The wire had a thin coating of amber varnish, and the insulation appeared to 

 be sufficient. In order to test this, a series of observations of R were taken when 

 the calorimeter was filled with pure pentane. The increase in E did not exceed 

 1 in 22,000. 



§ The mass of German silver used by us in the arm R weighed several pounds 

 and contained about 1800 feet of single wire in triple and double strands. 



