80 



CHEMICAL PHYSICS. 



induction coil increases with the length of its wire, which in large instruments 

 is fifty miles and more. While such machines are in operation sparks several 

 feet in length will pass between the terminals of the secondary coil at A. 



Conversion of electrical energy into heat and light. "When- 

 ever a current passes through a wire (or through any other mass) it 

 offers more or less resistance, the amount of which depends on the 

 nature of the material and the thickness of the wire. This resistance 

 gives rise to the conversion of electrical energy into heat. Practical 

 use is now extensively made of these heating effects by passing strong 

 currents through poor conductors, as is done in the heaters used for 

 heating cars or buildings, in stoves for cooking purposes, in the 

 vulcanizers used in dental operations, and in electric furnaces. In 



M 



Electric furnace. 



the latter, powerful currents are employed to produce temperatures 

 unattainable by processes of combustion or by any other means at 

 our disposal. These furnaces have not only revolutionized many 

 processes of manufacture, but have led to the discovery of a number 

 of substances. 



The construction of electric furnaces differs widely according to 

 the use made of them. Fig. 31 represents the vertical section of a 

 furnace used for the manufacture of aluminum-bronze. The material 

 to be acted on is contained in a graphite crucible, A, resting on the 



