THE BOLOMETER. 171 



wire 0.06 mm. diameter, it can be readily shown that the cross-section of 

 the thermopile is about 56 times that of the bolometer, and from their 

 heat conductivities, for the same temperature gradient, that the loss of 

 heat by conduction in the thermopile is about 100 times that of the bo- 

 lometer. But the temperature gradient at the ends of a bolometer strip 

 carrying an electric current may be 50 to ioo C, so that the heat lost by 

 conduction may be about the same for both instruments. Since the 

 temperature of the bolometer is from 50 to ioo C. higher than the ther- 

 mopile, the loss of heat per second due to radiation in the former is from 

 2 to 3 times that of the latter. But the mass of the thermopile is 5 times, 

 while its specific heat is 3.3 times, that of the bolometer. Hence, to raise 

 the temperature of thermopile and the bolometer to the same extent, 16 

 (5X3.3) times as many heat units must be supplied to the former. Since 

 the loss by radiation is 3 times as great from the bolometer, it will require 

 about 5 times as long for the thermopile to reach a steady temperature. 

 In practice, however, on account of the blackening of the surface, the 

 bolometer is not so quick in its action as here computed. 



From these considerations, as well as the mechanical difficulties in 

 constructing a thermopile of wire less than 0.05 mm. in diameter (and 

 keeping the resistance low) , it will be seen that the thermopile can not be 

 made so quick in its action as the bolometer and hence is not so well 

 adapted where a quick automatic registration of the galvanometer deflec- 

 tions is desired. But, as will be shown presently, since it is difficult to 

 read large deflections accurately in less than a 4 to 5 second single swing 

 of the galvanometer system, a thermopile of 0.06 to 0.08 mm. wire, which 

 attains a steady temperature in this interval of time, is not objectionable, 

 and, since it is less disturbed by air-currents (being at room temperature), 

 it may be the more reliable instrument (see table IV) . Neither instrument, 

 however, compares with the radiometer in steadiness. The amount of 

 work done on emission, absorption and reflection spectra, as well as the 

 accuracy attained, in the infra-red to 15 //, where the radiometer deflections 

 were, again and again, only a few tenths of a millimeter would not have 

 been possible with these instruments. In a recent examination of reflection 

 spectra of minerals, using a thermopile, the accuracy attainable without 

 repeating the readings several times was far from that of the radiometer, 

 although the actual deflections were larger. 



The present experimental comparison of the thermopile, of 0.08 mm. 

 iron and constantan wire, and the platinum bolometer was undertaken in 

 order to determine the accuracy attainable in measuring a constant source 

 of radiant energy, and hence to learn the feasibility of substituting the 

 thermopile for the troublesome bolometer. Within experimental error it 

 has been established by Langley, by Rubens and by Julius that the bolom- 

 eter (galvanometer) deflections are proportional to the current flowing 

 through the bolometer, and also to the amount of energy falling upon the 



