TIIE NATURE OF ELECTRICITY. 27 



circumstances, produce both heat and electricity, in 

 numerous instances; in others, equally numerous, both 

 heat and light, and in others, heat, light, and elec- 

 tricity. 



The simultaneous production of heat and electric- 

 ity is seen in the examples already given of bodies 

 electrified by friction, of which heat is also a neces- 

 sary result. 



Another prominent instance is the action of the 

 electric generators known as dynamos; in which the 

 evolution of heat is such that special provision for cool- 

 ing has to be made, to prevent injury. Here mechani- 

 cal action is the agent. 



In the galvanic battery we have a well-known in- 

 stance of the production of heat and electricity by 

 chemical action ; as a certain amount of heat, more or 

 less perceptible, is always a result. 



Instances of the simultaneous production of heat 

 and light are numerous and well known, as the heat- 

 ing of an iron rod, which becomes luminous when the 

 temperature rises to a certain degree ; whether it be 

 heated by friction, as of a shaft and journal, or by the 

 chemical action of a furnace. 



The dynamo and galvanic battery have been re- 

 ferred to as producing both heat and electricity. 

 When a current of this electricity, of sufficient in- 

 tensity, is passed through a conductor of high resist- 

 ance, as a fine platinum wire, or a carbon filament, they 

 become luminous by incandescence ; and when passed 

 through two sticks of carbon slightly separated, we have 

 light of great intensity ; and there is, in both instances, 

 the evolution of intense heat. These are perhaps the 

 most striking examples which can be given of the sim- 



