CHAPTER II. 

 RESISTANCE AND ELECTROMOTIVE FORCE. 



HEATING EFFECT OF THE ELECTRIC CURRENT. 



11. Electrical resistance. When a pump forces water through 

 a circuit of pipe, a part of the work expended in driving the pump 

 reappears as heat in the various parts of the circuit of pipe because 

 of the resistance which the pipe offers to the flow of water. 

 Similarly, when an electric generator produces an electric cur- 

 rent in a circuit, a part of the work expended in driving the 

 generator reappears as heat in the various parts of the circuit. 

 The current seems to be opposed by a kind of resistance * more 

 or less analogous to the resistance which a pipe offers to the 

 flow of water, and a portion of an electrical circuit is said to have 

 more or less electrical resistance according as more or less heat is 

 generated in it by a given current. 



12. The heating effect of the electric current. Joule's law. 



The amount of heat which is generated in a given wire is propor- 

 tional to the square of the current that is flowing in the wire and 

 to the time that the current continues to flow, that is, 



H=RPt (2) 



in which H is the amount of heat generated in a wire in / sec- 

 onds by a current of / amperes, and R is a constant for a 

 given wire. The value of this factor R is used as a numerical 

 measure of the electrical resistance of the wire. 



Practical applications of the heating effect. The heating effect 

 of the electric current is utilized in the various forms of electric 

 lamps in which a filament of carbon or refractory metal is heated 

 to brilliant incandescence by the electric current. The heating 



* An exact mechanical analogue of electrical resistance is given in Art. 62. 



2 5 



