VISIBLE AND NEAR-VISIBLE RADIATION 145 



radiation to the surroundings. This receiver unit C is clamped at the 

 center of a spUt housing AB. Each half of the housing is made of two 

 parts separated by a sufficient distance to secure electrical insulation. 

 Thus the assembled housing occupies practically the entire solid angle 

 surrounding the receiver. Radiation can be introduced only through the 

 narrow slit opening in A. The whole assembly is then placed in an 

 evacuated space, radiation being introduced through a suitable window, 



o 



quartz for radiation lying between 2000 A and 2.5 n, salt if the instrument 

 is to be used for the infra-red between 2.5 /j. and 15 /jl. Where such 

 extreme precautions have to be taken to avoid the influence of surround- 

 ing bodies upon the detector, it is necessary that the galvanometer and 

 leads also be maintained at constant temperature. Although the entire 

 electrical circuit may be made up of copper (except for the junction), 

 electromotive forces may arise from impurities (which result in inhomo- 

 geneities in the circuit) and from other similar causes. 



Wherever windows are used, their transmission characteristics must 

 be taken into account. For total radiation measurements, where some 

 of the radiation lies in the infra-red, bare thermocouples should be used. 

 Such thermocouples do not have the high sensitivity of the vacuum 

 thermocouple, owing to the greater heat loss by convection. The 

 method of shielding described is of even greater importance in the use 

 of such bare thermocouples. The sensitivity of such an instrument 

 depends upon : 



(a) The choice of alloys having the quantity 



E 



maximum 



where E = thermoelectric power. 

 Ki and K2 = heat conductivities. 

 (Ti and 0-2 = specific resistances of the lead wires, respectively. 



(6) The proper relation of size of receiver, length of wire, and resist- 

 ance of the couple. 



(c) The sensitivity of the current-measuring instrument. For most 

 purposes, a high-sensitivity d'Arsonval galvanometer is preferred. For 

 even greater sensitivity, amplification may be applied to the galvanom- 

 eter. For a more complete discussion, the reader is referred to the 

 hterature (12). 



RADIOMETERS 



Another type of instrument which has been used to some extent for 

 spectroscopic purposes is the radiometer (5). It consists of a suspension 

 with two symmetrical receivers, mounted on a light quartz fiber and 



