IMPEDANCE. 



47 



The lower curve in Fig. 21 is practically the magnetisation 

 curve of the coil, and resembles the magnetisation curve 

 discussed later in connection with transformers, &c. 



The lower curve is obtained by plotting the voltage at 

 the terminals of the coil instead of the impedance. 



2-5 



40 



30 



20 



10 



Amperes. 

 FIG. 21. VARIATION OF IMPEDANCE WITH CURRENT. 



10 



The voltage represented by this curve is almost entirely 

 due to the back electromotive force of self-induction referred 

 to on page 16, the drop in the resistance of the windings 

 being only very slight. For simplicity, we may assume the 

 curve to show the back electromotive force corresponding to 

 each value of the current in the coil. From the results it 

 would then be easy to calculate the number of magnetic lines 

 formed in the core. The formula giving the back voltage in 

 terms of the magnetic flux, which will be obtained later, is : 



