STANDARDS OF RESISTANCE. 317 



double spiral on the surface of a wooden cylinder, and ends in two 

 large metal rods. It is protected by a metal case, which leaves 

 sufficient space for the circulation of air, and the whole is enclosed 

 in a wooden box. A thermometer can be placed in a cavity in the 

 centre of the coil, but the determination of the temperature is more 

 difficult, since the instrument cannot be immersed in liquid. The 

 best plan is to surround it with a thick layer of wadding. 



925. There is at present no very exact information as to the 

 degree of permanence of standards of resistance. With mercury 

 the only changes to be feared are those arising from the deformation 

 of the glass ; the imperfect purity met with in commercial specimens 

 of mercury has no appreciable influence ; a fresh filling of the tube 

 does not change the results, and, except as regards the facility of the 



Fig. 1 80. 



operation, it appears immaterial whether the filling takes place in a 

 vacuum or in air ; finally, the temperature is easily determined. 



There is not the same certainty about solid metal standards. The 

 committee of the British Association deposited several standards at 

 Kew Observatory, either of platinum, or of alloys of platinum and 

 silver, of platinum and iridium, or of gold and silver. Matthiesson 

 and Hockin compared these standards in 1867, and determined the 

 temperatures at which they then had the same resistance.* 



A fresh comparison made in 1876 by Chrystal and Saunder,f 

 with very elaborate precautions as to the exact determination of 

 temperature, did not give the same results ; but it might be thought 



* British Association Report for 1867. Dundee. Reprint, p. 145. 

 t British Association Report for 1876, p. 1 3. Glasgow. 



