INDUCTANCE AND CAPACITY. 



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in which A^ is the small quantity of electricity which flows past 

 a given point on the wire during the short interval of time A/. 



Quantity of electricity is usually spoken of as electric charge 

 or simply as charge. 



Quantity of water is the fundamental and easily measured 

 thing in hydraulics, and water current is most conveniently 

 defined as quantity of water per second. On the other hand, 

 in the case of electricity, the fundamental and easily measured 

 thing is electric current and quantity of electricity is most con- 

 veniently defined as the product of electric current and time. 



Units of electric charge. The quantity of electricity transferred 

 in one second by one ampere is called a coulomb ; q in equation 

 (9) is expressed in coulombs when i is expressed in amperes and 

 / in seconds. 



9. Electrically charged bodies. Consider two metal bodies, 

 A and B, Fig. 3, which at a given instant are connected, as 

 shown, to the terminals of a battery, or to any source of electro- 

 motive force. When the wire 

 is connected a momentary pulse 

 of current flows through the 

 wire out of one body and into 

 the other, and the bodies A and 

 B are said to become charged 

 with electricity. The body into 

 which the momentary current 

 flows is said to become posi- 

 tively charged, and the body 

 out of which the momentary 

 current flows is said to become 

 negatively charged, that is, the 

 charge on one body is -f q and the charge on the other body 

 is q. Electrically charged bodies always occur thus in pairs, 

 the positive charge on one body being always associated with an 

 equal negative charge on some other body or bodies. 



Fig. 3. 



