CHAP, vii.] ELECTRICITY AND MAGNETISM. 329 



circulate for ever ; just as a waggon set rolling along a 

 circular railway should go round for ever if it were not 

 stopped by friction. When matter in motion is stopped 

 by friction the energy of its motion is frittered down by 

 the friction into heat. When electricity in motion is 

 stopped by resistance the energy of its flow is frittered 

 down by the resistance into heat. Heat, in fact, appears 

 wherever the circuit offers a resistance to the current. 

 If the terminals of a battery be joined by a short thick 

 wire of small resistance, most of the heat will be de- 

 veloped in the battery ; whereas, if a thin wire of con- 

 siderable resistance be interposed in the outer circuit, it 

 will grow hot, while the battery itself will remain com- 

 paratively cool. 



367. Laws of Development of Heat : Joule's 

 Law. To investigate the 

 development of heat by a 

 current, Joule and Lenz used 

 instruments on the prin- 

 ciple of Fig. 136, in which 

 a thin wire joined to two 

 stout conductors is enclosed 

 within a glass vessel con- 

 taining alcohol, into which 

 also a thermometer dips. 

 The resistance of the wire 

 being known, its relation to 

 the other resistances can 

 be calculated. Joule found 

 that the number of units of heat developed in a con- 

 ductor is proportional 



(i.) to its resistance ; 



(ii.) to the square of the strength of the current ; 

 and 



(iii.) to the time that the current lasts. 

 The equation expressing these relations is known as 

 Joule's Law, and is 



Fig. 136. 



