124 ADVANCED ELECTRICITY AND MAGNET1MS. 



force of 100 volts is sufficient to produce a very perceptible 

 attraction with this arrangement. 



76. Measurement of electric charge. The ballistic galva- 

 nometer. A very large amount of electric charge may be deter- 

 mined by observing the time during which the charge will main- 

 tain a sensibly constant measured current. Thus the discharge 

 capacity of a storage cell may be measured by observing the 

 time during which the cell will deliver a current of, say, 10 

 amperes. The amounts of charge which are most frequently 

 encountered in the momentary flow of electric current in open 

 circuits are, however, exceedingly small; and it is evident that 

 a very small amount of electric charge cannot be measured by 

 the method above suggested. Such small electric charges are 

 measured by means of the ballistic galvanometer. This is a 

 galvanometer usually of the D'Arsonval type with a fairly heavy 

 moving coil. When a momentary pulse of current flows through 

 such a galvanometer, the coil is set in motion, and a certain 

 maximum deflection or throw of the coil is produced. Let d be 

 the measure of this maximum deflection or throw on the galva- 

 nometer scale. Then we have: 



a = kd (i) 



in which g is the amount of electric charge carried through the 

 galvanometer coils by the momentary pulse of current, and k 

 is a constant for the given galvanometer. The value of the factor 

 k is generally determined in practice by sending through the 

 galvanometer a known amount of charge and observing the throw 

 d produced thereby.* 



77. The capacity of a condenser. A ballistic galvanometer 

 BG, a condenser and a number of dry cells are connected as 

 shown in Fig. 83. One terminal of the condenser is connected 

 to a flexible wire which is fixed to the end of a glass handle. By 

 touching the wire W to the point 6, the electromotive force E 



* The use of a ballistic galvanometer is discussed at some length in Absolute 

 Measurements in Electricity and Magnetism by Andrew Gray, Vol. 2, pages 391-397- 



