38 FEAGMENTS OF SCIENCE. 



heat falls upon the anterior face of a thermo-electric 

 pile,* 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 falling upon the pile. This 

 famous instrument, and not an ordinary thermometer, 

 is what we shall use in these enquiries, but we shall use 

 it in a somewhat novel way. As long as the two oppo- 

 site 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 conse- 

 quence of a difference of temperature between the two 

 opposite faces of the pile. Hence, if after the anterior 

 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 neutralise the 

 former. When the neutralisation is perfect, the mag- 

 netic needle connected with the pile is no longer de- 

 flected, 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 acting at the same time on the two 

 opposite faces of the pile. When by means of an ad- 

 justing screen, perfectly equal quantities of heat are 

 imparted to the two faces, the needle points to zero. 

 Let any gas be now permitted to enter the exhausted 

 tube; if its molecules possess any power of intercepting 

 the calorific waves, the equilibrium previously existing 

 will be destroyed, the compensating source will tri- 



* In the Appendix to the first chapter of ' Heat as a Mode of 

 Motion,' the construction of the thermo-electric pile is fully ex- 

 plained. 



