ELECTRICAL MEASUREMENTS. 285 



through the pivoted coil A (this current is equal to ^/^), and 

 the force action between the two coils causes the coil A to move 

 and carry a pointer over a divided scale. The force action be- 

 tween the two coils is proportional to the product of the currents 

 in the respective coils, that is, the force action is proportional to 

 e/R x i or proportional to ei since R is constant. But ei 

 is the power delivered to the receiving circuit, and therefore, 

 since the force exerted on the pointer is proportional to the 

 delivered power, the scale over which the pointer plays may be 

 divided and numbered so as to indicate watts of power directly. 

 The wattmeter is always used for the measurement of power 

 delivered by an alternator. 



USE OF THE BALLISTIC GALVANOMETER. 

 162. Measurement of electric charge and of magnetic flux by 

 means of the ballistic galvanometer. (a) When a ballistic gal- 

 vanometer is used to measure the discharge q from a condenser, 

 the charged condenser is connected to the galvanometer termi- 

 nals, and the throw d of the galvanometer is observed. Then 



q=kd (i) 



in which k is a proportionality factor which is called the reduc- 

 tion factor of the galvanometer. This reduction factor is gener- 

 ally determined by observing the throw d produced by a known 

 amount of charge q. Thus a condenser of known capacity C 

 may be charged by a known electromotive force E and dis- 

 charged through the galvanometer, giving q = EC=kd from 

 which k may be calculated. 



(b) The ballistic galvanometer is frequently used to measure 

 what is called the impulse value of the momentary electromotive 

 force which is induced in a coil of wire during the time that the 

 magnetic flux through the coil is changing by a certain amount. 

 Thus, a coil containing Z turns of wire is placed in a magnetic 

 field so that a certain amount of magnetic flux 4> * passes through 



* This flux (f> represents the flux through a mean turn of wire on the coil. 



