THERMOPILES. 1 57 



that the sensitiveness of these thermopiles did not increase appreciably in 

 a vacuum is rather remarkable. Brandes (Phys. Zeit, 6, p. 503) found 

 that a single junction of 0.02 mm. wire became 18 times more sensitive in 

 a vacuum. Lebedew (Ann. der Phys., 9, p. 209) found that a 0.025 mm ' 

 iron-constantan junction when black was 7 times, and when bright 25 

 times more sensitive at a pressure of 0.01 mm. than for atmospheric pres- 

 sure. 



The Peltier Effect. 



The result of the Peltier effect is to lower the temperature of the ex- 

 posed junction. Consequently, the thermopile does not give an accurate 

 record of the energy received. The actual error introduced has never 

 been determined. Since there is a possibility of using the thermopile for 

 quantitative work in place of the bolometer, it is desirable to learn the 

 degree of accuracy of this instrument. 



The rate of generation of heat by the Peltier effect is proportional to 

 the current, while the generation of heat on account of resistance is pro- 

 portional to the square of the current. Jahn 1 has shown that the heat 

 generated by the Peltier effect, determined experimentally, agrees, within 

 experimental error, with the value computed from the observed thermo- 

 electric power. The value for iron-constantan has never been determined 

 experimentally 2 but from the work of Jahn it is permissible to compute 

 the heat generated in the thermopile by using the known thermoelectric 

 power which is about 50X10"" 8 volts. 



Using a galvanometer of 5 ohms resistance and having a figure of 

 merit of 2 = 3 Xio -10 ampere per millimeter for a scale at 1 m., and an 

 iron-constantan thermopile of 20 junctions, wire 0.08 mm. and 5 ohms 

 resistance, 1 mm. = 2Xio -8 C. The Peltier effect in calories is computed 

 from the formula 



p= _TU dE 

 J ' dt 



where 7^=274; 2=3 Xio -11 c.g.s.; /=5 sec; 7=4.2Xio -7 and dE/dt= 

 50X10 -2 c.g.s. units; 



P= 5Xio~ 12 gr. cal. 



The total weight of the junctions is about 0.01 gr. and the specific 

 heat is about 0.1 gram calorie. Hence the temperature change of the 

 exposed junctions is: 



M= 5Xl =cXio- fl (for 1 mm. deflection) 

 0.1X0.01 3 v 



1 Jahn: Wied. Ann., 34, p. 755, 1898. 



2 Since writing this, it has been found that Lecher (Ber. Akad. Wiss. Wien, 115, p. 1505, 

 1906; Sci. Abstracts, 1083, 1907) has recently determined this constant to be 12.24 g r - ca ^ 

 per amp. hr., while the value previously computed was 10.5 gr. cal. per amp. hr. 



