THE BOLOMETER. 165 



that of a bolometer, hence the radiometer must be sensitive to temperature 

 changes less than the value just given. 



The sensitiveness of bolometers thus far attained is about 1 00 q 000 

 degree per millimeter deflection. Paschen claims a sensitiveness of 

 10 0(H) (T oo degree by reading to o.i mm. But the conditions are rare 

 when one can read to o.i mm., so that the estimate would seem too high. 

 As will be seen presently, the working sensitiveness is of the order of 

 x 00 q 000 degree, or generally considerably less. 



V. THE BOLOMETER WITH ITS AUXILIARY GALVANOMETER. 



We have now to consider one of the most useful radiation meters yet 

 devised, viz, the bolometer which is simply a Wheatstone bridge, two 

 arms of which are made of very thin blackened metal strips of high elec- 

 trical resistance and high temperature coefficient, one or both of which 

 are exposed to radiation. When thus exposed, their temperature changes, 

 thus unbalancing the bridge, and the resulting deflection of the galvanom- 

 eter gives a measure of the energy absorbed. The maximum sensitive- 

 ness of the bolometer is limited by the size of the strip and is proportional 

 to the square root of the surface exposed to radiation. Any further gain 

 in sensitiveness must be attained by increasing the sensitiveness of the 

 galvanometer, which, for the moving magnet type, varies as the square of 

 its (undamped) period. The sensitiveness is also proportional to the 

 bolometer current, which is limited by the resistance of the bolometer 

 strips. 



It will be noticed presently that the working sensitiveness of the vari- 

 ous galvanometers thus far used is of the order of 2X10 -10 amp. per 

 millimeter deflection, while the temperature sensitiveness varies from 

 5Xio -5 to 5Xio~ 8 for 1 millimeter deflection for a scale at 1 meter. 



Historical. 



The various bolometer-galvanometer apparatus will first be noticed, 

 in so far as it relates to spectro-radiometric work. 



The first great step in improving the moving magnet galvanometer is 

 due to Kelvin who decreased the weight of the moving parts to a few 

 milligrams, and introduced the astatic system of magnets. The main 

 problem in bolometer construction is to use strips of a metal having a high 

 resistance-temperature coefficient, a small specific heat, and low heat con- 

 ductivity. These metals are nickel, platinum, tin, and iron, but for vari- 

 ous reasons in mechanical construction, platinum is the most commonly 

 used. 



The manner in which this instrument has been developed to its pres- 

 ent high sensitiveness is best illustrated by considering the various designs 

 of different investigators, given in table VII. 



