174 SCIENTIFIC APPARATUS. 



circuit, causes the circuit to be traversed by a current opposite 

 to that which would have produced the changes in question. 

 On the other hand, no current is produced by either supplying 

 heat to, or taking heat from, any homogeneous parts of a conduct- 

 ing circuit, nor by moving one part of such a circuit relatively to 

 another part. In fact, all the various methods by which electric 

 currents can be produced, are processes in which some reversible 

 effect of such currents is brought about by the expenditure of 

 external energy. 



G. CAREY FOSTER. 



