292 Messrs. J. H. Gray and J. B. Henderson. 



reading the knife-edge of B, the result could be easily obtained 

 correct to ^ per cent. The method being a zero one, the galvano- 

 meter could be made almost unstable, and, as there are over 23,000 

 turns on the coil, the arrangement was exceedingly sensitive. The 

 maximum current used was 0'5 ampere, and, in the case of iron and 

 steel wire, not more than 0'25 ampere. 



The four resistances r lt r 2 , r 3 , and r 4 , of 150 ohms each, were care- 

 fully wound anti-inductively side by side on a piece of slate, and 

 covered with cotton tape, to ensure their being at the same tempera- 

 ture. The whole system of wires was so arranged as not to influence 

 the galvanometer. The distance between the standard and test wire 

 being so small, the temperatures of both were the same, and the 

 sliding contact, at the point where it had to be touched by the hand, 

 was protected by a piece of vulcanite from being heated. With these 

 precautions, no inconvenience was experienced from thermo-electric 

 currents. 



Calculation of Results. 



From the method of calculation it will be seen that it was only 

 necessary to measure lengths in order to obtain results of the varia- 

 tions of the weight specific resistance, and to determine densities in- 

 addition, when the volume" resistance was also required. The 

 measurement of so great lengths as 400 cm. could be made very 

 accurately, and, therefore, very little error was introduced. In no- 

 case was it necessary to measure the section in connexion with the 

 resistance. 



Let I = original length of the test wire which would give a balance 

 with the standard, I' = the length which would give a balance after 

 applying weight to it, L = the original length between B and M 

 before applying weight, iv = the section before applying weight,. 

 L', n/ the corresponding values after applying weight, 



<T V = volume specific resistance, 

 a w = weight specific resistance, 

 R, = resistance between the points P and Q on the standard. 



Then, since the sliding contact on the test wire is always adjusted 

 till the resistance is equal to Bj, we have 



I , 



a v p = a vf j _ T _ , 

 tap iv ft 



where /> = density before applying weight, 



= after 



