CHAPTER VI.* 



ELECTRIC CHARGE AND THE CONDENSER. 



74. Electric charge. A current of water through a pipe is a 

 transfer of water along the pipe. Let q 'be the amount of 

 water which, during / seconds, flows past a given point in a 

 pipe, then the quotient q/t is the rate of flow of water through 

 the pipe, and this rate of flow may be spoken of as the "strength " 

 i of the water current. Therefore, if i is the strength of water 

 current (units of volume per second), then the amount of water 

 flowing past a given point of the pipe in / seconds is given by the 

 equation 



2 = it 



Similarly, an electric current in a wire may be looked upon as a 

 transfer of electricity along the wire, and the quantity q of 

 electricity which flows past a point on the wire during / seconds 

 may be defined as the product of the strength of the current 

 and the time. That is: 



g = it (i) 



If the strength of the current is variable, then equation (i) must 

 be written in the form : 



Ag = i-At (2) 



in which Aq is the amount of charge which flows past a given 

 point on the wire during a short interval of time At. 



Volume of water is the fundamental and easily measured thing 

 in hydraulics, and water current is most conveniently defined 

 as cubic feet of water per second. In the case of electricity, 

 however, the fundamental and easily measured thing is electric 



* The usual elementary treatment of electrostatics is not sufficient as an intro- 

 duction to Chapters VII and VIII. Therefore a very brief outline is given in this 

 chapter. The student is urged to read pages 125-171 of Franklin and MacNutt's 

 Elementary Electricity and Magnetism. 



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