414 ELECTRICAL MEASUREMENTS 



nitude depending on the square of the frequency, hence its in- 

 creasing importance at high periodicities. This shows that all 

 massive metal frames and metal fastenings must be avoided. 

 The coils must be wound with a conductor made up of small 

 strands which are insulated from one another. For economy of 

 space, enameled wire may be used. 



Wilson Method for Measuring Inductance. In this method 

 the reactive component of the potential difference between the 

 terminals of the unknown inductance is measured by a quadrant 

 electrometer. 25 The connections are shown in Fig. 241. 



The two sets of quadrants are connected to the terminals 

 of the unknown inductance. One end of the needle circuit is 

 attached to R x , preferably at the middle. T is an air-core trans- 

 former of mutual inductance, m, and V an electrostatic voltmeter 



A m < - d - ^ 



FIG. 241. Connections for Wilson method for measuring inductance. 



for determining the potential of the needle. A is an ammeter 

 for measuring the main current. 



When applied to this case, the elementary formula for the 

 deflection of the quadrant electromotor becomes 



d = R x i + L x -fa- 



Let the readings of the electrostatic voltmeter and of the 

 ammeter be V and 7. Assuming sinusoidal currents, 



L 



di 2 



/\ -i 



(sM 



