SUMMARY. 



175 



the galvanometer was at its full sensitiveness no resistance in series 

 so that in the actual experiments (table IX) the error was less exaggerated. 

 For the 240 disk where the error in the deflections was only 1 or 2 mm. 

 the correction reduces the observed values close to the true one. 



From the consistency of the ratios in each series, which is of the order 

 of 0.2 to 0.3 per cent, it will be noticed that the bolometer is a very reliable 

 instrument in spite of its mechanical weakness. 



6 8 10 IZ 14- IQ 18 .20 



Fig. 105. Radiation due to rotating sectored disk. 



24 26C777 



VI. SUMMARY. 



The present paper deals with four instruments for measuring radiant 

 energy, viz, the radiomicrometer, the linear thermopile, the radiometer, 

 and the bolometer with its auxiliary galvanometer. 



As a result of this historical inquiry and by experiment it was shown 

 that the radiomicrometer is capable of great improvement, by reducing 

 its weight, by lengthening its period, and by placing it in a vacuum. It 

 was further shown that on account of para- and dia-magnetism the sensi- 

 tiveness of the radiomicrometer is very limited, perhaps only a fifth of the 

 best bolometers described. 



It was also shown that the Rubens thermopile is as sensitive as the 

 best bolometer, and that its heat capacity can be greatly reduced by using 

 thinner (0.06 to 0.08 mm. diameter) wires, which are made shorter, thus 

 keeping the resistance low. The computed errors due to the Peltier effect 

 are about 1 part in 300. The thermopile is not so well adapted as is the 

 bolometer for instantaneous registration of radiant energy and it does not 

 admit so great a range in variation of sensibility; but, on account of its 

 greater steadiness, it commends itself for measuring very weak sources of 

 radiation, e.g., the extreme ultra-violet and infra-red region of the spectrum. 



By a direct comparison it was shown that the radiometer can be made 

 just as sensitive as the bolometer, but its period will be much longer, It 



