Comparison of Platinum Thermometers. 



423 



was one of the type described by Callendar (Phil. Mag. July 

 1891) with coils on the binary scale, standardized in the usual 

 manner. The contact surfaces of the screw-nuts used for 

 short-circuiting the coils were kept clean, and the contacts 

 frequently tested. The galvanometer u*ed was one specially 

 designed by Prof. Callendar (Phil. Mag. Feb. 1892) for 

 resistance pyrometer work, with an astatic-needle system in 

 which the control is due to the passage of a current through 

 a pair of coils, marked " Field" in fig. 1, at right angles to 

 the position of the galvanometer-coils. The current which 

 effects the control is proportional to that used in measuring 

 the resistance, and hence the deflexion due to any given 

 difference of resistance is constant, no matter what the 

 current may be. It was found, for example, that the deflexion 

 for a difference of resistance of one unit of the bridge, was 

 exactly the same whether a two-volt or four-volt battery was 

 used. 



In order to secure a proper sensitiveness of the needle, the 

 field or control coils were shunted with a small resistance 

 marked Sh, which was adjusted so that a change of resistance 

 of one bridge-unit produced a deflexion equal to 10 divisions 

 of the galvanometer-scale. By this means, the deflexions 

 could be read to 1/100 of a degree, which was sufficient for 

 the purpose. 



2 G2 



