RADIATION 43 



glass cylinder, three feet long and three inches wide. The 

 two ends of this cylinder are stopped by two plates of 

 rock-salt, a solid substance which offers a scarcely sensi- 

 ble obstacle to the passage of the calorific waves. After 

 passing through the tube, the radiant heat falls upon the 

 anterior face of a thermo-electric pile, 1 which instantly 

 converts the heat into an electric current. This current 

 conducted round a magnetic needle deflects it, and the 

 magnitude of the deflection is a measure of the heat fall- 

 ing upon the pile. This famous instrument, and not an 

 ordinary thermometer, is what we shall use in these in- 

 quiries, but we shall use it in a somewhat novel way. As 

 long as the two opposite faces of the thermo-electric pile 

 are kept at the same temperature, no matter how high that 

 may be, there is no current generated. The current is a 

 consequence of a difference of temperature between the 

 two opposite faces of the pile. Hence, if after the ante- 

 rior face has received the heat from our radiating source, 

 a second source, which we may call the compensating 

 source, be permitted to radiate against the posterior face, 

 this latter radiation will tend to neutralize the former. 

 When the neutralization is perfect, the magnetic needle 

 connected with the pile is no longer deflected, but points 

 to the zero of the graduated circle over which it hangs. 



And now let us suppose the glass tube, through which 

 the waves from the heated plate of copper are passing, to 

 be exhausted by an air-pump, the two sources of heat act- 

 ing at the same time on the two opposite faces of the pile. 

 When, by means of an adjusting screen, perfectly equal 



1 In the Appendix to the first chapter of "Heat as a Mode of Motion," the 

 construction of the thermo-electric pile is fully explained. 



