sensitiveness in Detectors of Radiant Heat. 71 



For practical reasons, however, it is usual to make a side of 

 the bridge adjacent to 6 equal in resistance to it. Putting there- 

 fore either b/c = 1 or <y/6 = 1, the other of the pair of fractions is 

 to be made as small as possible, and then 



4 



The resistance of the galvanometer should be 6/2 in the first case, 

 and 26 in the second. 



6. For most metals n = '0038 and, as before, 7e = '000252, 

 hence 



8 = 085XH \/A0 o , 

 in the case of maximum sensitiveness, or 



8 = -0G0\H\/A6 

 in the practical case. 



The condition that the bolometer is as sensitive as the 

 thermopile of 2 cms. length which is discussed above, is in the 

 first case = 1, or the strip is raised 1° C by the passage of the 

 current. In the practical case of equal arms in the bridge, the 

 strip must be raised 2° C. for equal sensitiveness. The sensitiveness 

 of the bolometer can be improved by increasing r) or decreasing k. 

 Thus we may expect a great gain by placing the bolometer in an 

 evacuated vessel, because not only is k decreased, but also the 

 irregularities of temperature due to convection currents are 

 decreased, and hence we may increase O . 



