IMPEDANCE 



43 



It is assumed that the con r in this case has a movable 

 core, or is arranged so that its magnetic circ lit can be varied 

 by alteration of the air gap in it. 



Instructions. Make connections exactly as described for 

 Experiment IV. The current must in this case also be kept 

 approximately constant if the effect of alteration of the mag- 

 netic circuit is to be considered alone. The current must, as 

 before, be regulated by adjusting the resistance R. The 

 frequency must also remain constant. 



For a series of positions of the movable part of the 

 magnetic circuit take readings of the current and of the 

 voltage across the terminals of the inductive winding. From 

 these readings calculate in each case the impedance as in 

 the previous experiment, and tabulate the results. 



The alteration in the magnetic circuit should be carried 

 out in such a way that an equal movement of the moving 

 portion is made between each pair of readings ; the observa- 

 tions may then be plotted as a curve with displacement 

 measured horizontally and impedance plotted vertically. 



DETERMINATION OF IMPEDANCE WITH VARYING AIR GAP. 



Description of coil Resistance -15 ohms. 



Current employed 3 amps, at 30 periods. 



A set of readings taken from a choking coil with a movable 

 laminated core are reproduced in the form of a curve shown 

 in Fig. 20. 



The coil employed was the same as that referred to in 

 ths last two experiments. The air gap was varied by drawing 

 out the core, leaving the measured gap between the end of 

 the core and a projection on the base, which completed the 

 magnetic circuit when the core was pushed home. 



The value of the coefficient of self-induction for each 

 position of the movable core may be calculated from the 



