122 ADVANCED ELECTRICITY AND MAGNETISM. 



current, and quantity of electricity is most conveniently defined 

 as the product of strength of electric current and time. 



Units of electric charge. Quantity of electricity is usually 

 spoken of as electric charge or simply as charge. The ampere- 

 second is the amount of charge which flows in one second through 

 a wire which carries a current of one ampere. The ampere-second 

 is usually called the coulomb. One ampere-hour is the amount of 

 charge flowing in one hour through a wire carrying one ampere. 

 The ampere-hour is extensively used among electrical engineers 

 in specifying the discharge capacity of storage batteries. The 

 abcoulomb is the amount of charge which flows in one second 

 through a wire carrying a current of one ab-ampere. One ab- 

 coulomb is equal to ten coulombs. 



75. The momentary flow of current in an open circuit. Elec- 

 trically charged bodies. Figure 81 shows a battery maintaining 



Fig. 81. 



an electric current through a circuit which contains a coil of 

 wire on an iron core, and the circuit can be broken at will at the 

 point p. Two metal plates A and B with a thin layer of 

 insulating material DD between them are connected as shown 

 in the figure. When the plates AB are not connected, a spark 

 is produced when the circuit is broken at p, showing that the 

 current continues to flow for a short time after the metallic 

 contact at p is broken. When the plates A and B are con- 

 nected, however, there is no spark at p when the circuit is 

 suddenly broken. In this case the current does not continue to 

 flow across the break at p ; it flows into plate A and out of plate 

 B, and the two plates A and B are said to become electrically 

 charged. The plate into which the momentary current flows is 



