408 Mr. Ervin S. Ferry on the Specific 



are in resonance with the primary, and hence with each other, 

 the principle of the bolometer is employed. 



Fig. 1. 



In the conventional diagram of the Wheatstone bridge 

 shown in fig. 2, if be and dc are of the same resistance, tbe 



Fisr. 2. 



galvanometer will show no deflexion on pressing the key k 

 when the resistance of ab equals the resistance of ad. If ah 

 and ad are of equal resistance at any given temperature and 

 have the same mean temperature-resistance coefficient, the 

 resistance of one will equal the resistance of the other, what- 

 ever the temperature. In other words, if no deflexion of 

 the galvanometer-needle is produced on pressing the key k, 

 ab and ad are at the same temperature, so long as the equality 

 of resistance of the other two arms of the bridge remains 

 unaltered. This latter equality is secured by winding the 

 two wires be and dc close together and parallel to each other 

 on the same spool. In fig. 1, one side of each of the secondary 

 circuits a'b and a"d consists of a piece of No. 36 iron wire, 



