ELECTRIC CHARGE. THE CONDENSER. l6l 



quantity of electricity is most conveniently defined as the product 

 of electric current and time. 



Units of electric charge. The ampere-second is the amount 

 of electricity 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 quantity of elec- 

 tricity 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 quantity of electricity which flows in one second 

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

 coulomb is equal to ten coulombs. 



86. Measurement of electric charge. The ballistic galvanom- 

 eter.* A very large electric charge may be determined by ob- 

 serving the time during which the charge will maintain a sensibly 

 constant measured current. Thus a given storage battery can 

 maintain a current, say, of 10 amperes for 8 hours, so that the 

 discharge capacity of the storage battery is equal to 80 ampere- 

 hours. The charges most frequently encountered in practice, 

 however, are too small to be measured in this way, and for such 

 charges the ballistic galvanometer is used as follows : 



The charge .to be measured is sent through a galvanometer in 

 the form of a pulse of electric current of very short duration. 

 This pulse of current sets the needle of the galvanometer swing- 

 ing. The maximum deflection d of the needle at the first 

 swing is called the throw of the galvanometer, and this throw, if 

 it is not too large, is proportional to the amount 6f charge q 

 which is carried through the galvanometer by the bulse of cur- 

 rent. That is, 



q = kd ^ (59) 



the quantity k is called the reduction factor of the galvanometer, 

 and it is usually determined in practice by observing the throw 

 produced by a known charge. Equation (59) is true foragalva- 



* See Chapter X for a more complete discussion of the ballistic galvanometer and 

 of the measurement of electric charge. 

 12 



