THERMOPILES. 



155 



with which just as much time is lost by repeating observations which may 

 be affected by the lack of constancy of the zero. The instrument is self- 

 contained, and where the greatest sensitiveness is not required, it deserves 

 a wider application. 



Table IV. Sensitiveness of Radiomicrometers and Rubens Thermopile. 



In table IV are given the various radiomicrometers thus far described 

 and their sensitiveness, which is expressed in centimeter deflections per 

 millimeter of exposed vane, for a candle and scale each at a distance of 

 1 meter. 



III. THE THERMOPILE. 



From a historical point of view the thermopile has been in use from the 

 very beginning of radiant energy measurements, and in the hands of Tyndall 

 and other pioneers in this domain rendered excellent service in spite of its 

 great heat capacity. For spectro-radiometric work, however, only the linear 

 thermopile of Rubens 1 is well adapted. This thermopile consists of 20 junc- 

 tions of iron and constantan wires about 0.1 mm. to 0.15 mm. thick (resist 

 3.5 ohms), and when used with a galvanometer, having a figure of merit of 

 i= 1.4X io -10 amp. (period = 14 sec), a deflection of one scale division indi- 

 cated a temperature change of i.iX io -8 . A candle at 5 m. gave a deflec- 

 tion of about 10 cm. or 250 cm. at 1 m. The area of the exposed face is 

 about 0.8X20 mm. The deflections were as rapid as a bolometer, and its 

 stationary temperature was reached in less time than the single swing of 

 the galvanometer needle. In other words, its heat capacity was so small 

 that it gave an accurate register of the energy falling upon it. In another 

 experiment, using a galvanometer sensitiveness of 2 = 5Xio~ 10 ampere, and 

 the scale at 1 m., 1 mm. deflection = 2. 2Xio~ 6 C. The sensitiveness is 

 the same as that of the best bolometers yet constructed, while its simplicity 

 commends itself even in spectrum radiation work. 



The general experience in this country, however, has been that the 

 commercial instrument does not fulfill all the excellent qualities claimed 

 for the one originally described. The wires are heavier than in the original 

 specifications, which makes the instrument sluggish. 



1 Rubens: Zs. fur Instrumentenkunde, 18, p. 65; 1898. 



