286 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



the coil. The coil is connected to a ballistic galvanometer, then 

 quickly removed from the field, and the galvanometer throw d is 

 observed. This throw is proportional to the product Z<&, so 



that we may write 



= k'd (ii) 



in which k' is a constant for a given value of the resistance of the 

 galvanometer circuit, and it is to be determined by observing the 

 throw produced by a known value of Z3?. If the resistance of 

 the galvanometer circuit is changed, the value of k' is altered. 



The value of ZQ in the above discussion is the impulse value 

 of the electromotive force which is induced in the coil of wire 

 during the time that it is being withdrawn from the magnetic 

 field. Let / be the short interval of time which elapses during 

 the movement of the coil. Then the flux through the coil 

 changes from <I> to zero during t seconds, the average rate of 

 change of flux is <&/(, the average value of the electromotive 

 force which is induced in the coil is Z<&lt, and the product of 

 this average electromotive force and the time is equal to Z4>. 

 The product of the average value of the electromotive force and the 

 time during which the electromotive force continues to act is called 

 the impulse value of the electromotive force. 



163. Measurement of capacity* The simplest method of 

 measuring the capacity of a condenser is to charge the condenser 

 by a known electromotive force, discharge it through a ballistic 

 galvanometer of which the reduction factor k is known, and ob- 

 serve the deflection d which is produced. Then q = kd = CE 

 from which C may be calculated. 



* The most accurate method for measuring the capacity of a condenser is to use a 

 rapidly rotating commutator-device arranged to charge the condenser a known number 

 of times per second from a battery of known electromotive force and discharge the con- 

 denser the same number of times per second through an ordinary galvanometer, the 

 steady deflection of which measures the average value of the current. The most accu- 

 rate method for determining the ratio of the capacities of two condensers is by means 

 of Wheatstone's bridge, as described in Practical Physics, Franklin, Crawford and Mac- 

 Nutt, Vol. 2, page 133. A method for measuring the ratio of the inductances of two 

 coils by means of Wheatstone's bridge is described in Practical Physics, Franklin, 

 Crawford and MacNutt, Vol. 2, page 129. 



