36 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



work is expended in maintaining the current is proportional to the 

 current, according to equation (6). 



Hydraulic analogue of electromotive force. An electric gen- 

 erator, such as a voltaic cell or a dynamo, is analogous to a cen- 

 trifugal pump, or fan blower, which develops a definite difference 

 of pressure between its inlet and outlet. Imagine a fan blower 

 connected to a circuit of pipe which goes out from the outlet and 

 returns to the inlet. The volume of air per second forced through 

 this pipe may be called the strengtJi of the air current, and the 

 rate at which the fan delivers energy in the maintenance of this 

 air current is equal to the product of the strength of the air cur- 

 rent and the pressure difference between inlet and outlet of the 

 fan. Let / be the strength of the air current (volume of air 

 flowing per second) and let E be the pressure difference between 

 inlet and outlet. The power developed by the fan in maintaining 

 the flow of air is 



P=EI 



This equation is identical to equation (6), and the pressure differ- 

 ence between inlet and outlet of the fan blower is exactly analogous 

 to what is called the electromotive force of an electric generator. 



Note. The power delivered by a fan to a circuit of pipe is not strictly propor- 

 tional to the volume of air delivered per second because an increased flow of air usually 

 causes a'slight decrease in the speed of the fan. Similarly, the power delivered to a 

 circuit of wire by a voltaic cell or dynamo is not strictly proportional to the strength 

 of the current because an increase of current usually causes a decrease in the electro- 

 motive force of the cell or generator. This decrease of electromotive force of a voltaic 

 cell is called polarization and it is discussed in Art. 22. The decrease of electromo- 

 tive force of a dynamo due to increase of current output is generally due to a slight 

 decrease of speed or to a weakening of the field magnet, or to both. 



Definition of the volt. When P in equation (6) is expressed 

 in watts (joules of work per second) and / in amperes, then E 

 is expressed in terms of a unit which is called the volt. That is 

 to say, the electromotive force of an electric generator in volts is 

 equal to the power in watts delivered by the generator divided 

 by the current in amperes, or in other words, the power deliv- 

 ered by a generator in watts is equal to the current delivered by 

 the generator in amperes multiplied by the electromotive force of 



