328 ELEMENTARY LESSONS ON [CHAP. vn. 



CHAPTER VII. 



HEAT, LIGHT, AND WORK, FROM ELECTRIC CURRENTS. 



LESSON XXXI. Heating Effects of Currents. 



366. Heat and Resistance. A current may do 

 work of various kinds, chemical, magnetic, mechanical, 

 and thermal. In every case where a current does work 

 that work is done by the expenditure of part of the energy 

 of the current. We have seen that, by the law of Ohm, 

 the current produced by a given battery is diminished in 

 strength by anything that increases the external resistance. 

 But the strength of the current may be diminished, in 

 certain cases, by another cause, namely, the setting up 

 of an opposing electromotive force at some point of the 

 circuit. Thus, in passing a current through a voltameter 

 (Art. 214) there is a diminution due to the resistance of 

 the voltameter itself, and a further diminution due to the 

 opposing electromotive -force (commonly referred to as 

 " polarisation ") which is generated while the chemical 

 work is being done. So, again, when a current is used to 

 drive an electromagnetic motor (Art. 375), the rotation 

 of the motor will itself generate a back -current, which 

 will diminish the strength of the current. Whatever 

 current is, however, not expended in this way in external 

 work, is frittered down into heat^ either in the battery or 

 in some part of the circuit, or in both. Suppose a 

 quantity of electricity to be set flowing round a closed 

 circuit. If there were no resistance to stop it .it would 



