40 



IMPEDANCE. 



Jfj Jf 2 . Source of alternating current. 



r. Inductive resistance or coil of which impedance 



is to be measured. 

 R. Adjustable resistance. 

 A . Ammeter reading current in circuit. 

 V. Voltmeter reading voltage across inductive 



resistance. 



Instructions. Connect to the terminals of the alternator 

 an inductive resistance or coil in series with an adjustable 

 resistance and ammeter. 



Connect a voltmeter to the terminals of the coil. 

 Take readings on ammeter and voltmeter for a number of 

 different speeds of the alternator. At each speed before taking 

 readings, adjust the current approximately to a fixed value, 

 which should be maintained throughout the experiment. 

 Note the speed of the alternator in each case, and calculate 

 the frequency thus : 



Frequency = revs, per minute x No. of pairs of poles of 

 alternator -j- 60. 



The readings should be tabulated thus : 



Determination of Dependence of Impedance on Frequency. 



Description of coil Cast iron base, movable core. 



Resistance -15 ohms. 



Current 3 amps. 



A curve should be plotted with frequency measured 

 horizontally and impedance vertically. 



Fig. 19 shows two curves obtained in the manner just 

 described. The lower curve gives the values obtained for 

 the impedance of an arc lamp choking coil. The ohmic 

 resistance of the coil was *15 ohm, so that nearly all the 

 impedance was due to its self-induction. That is, the 



