42 IMPEDANCE. 



two elements, resistance and reactance. The resistance 

 is independent of frequency ; the reactance depends directly 

 on the frequency. The frequency of the current supplied 

 determines the relative importance of the two elements. 



It would be easy to calculate the value of the self-induction 

 of the resistance from the readings shown on the curve. 



If the ohmic resistance is known, this can be done directly 

 from a single observation. 



If the resistance is not known, two points on the curve will 

 enable the calculation to be made. 

 Thus the upper curve shows 



at frequency 10 impedance = 2-35 



40 - 5-3 



In 1st case R~ + (2 TT n L)- = (2-35) 2 = 5-5 

 2nd IP + (2 * n, L)' 2 = (5-3) 8 - 28-0 

 .-. By subtraction (2 * L)- (n\ - ri") = 22-5 



_ 22-5 22-5 Q 



: (nj-n 2 )4^~ " 15UO x 4 x 9'87 = 

 . . L = -0195 henries, 



or 19'5 millihenries. 



The determination of the effect of self-induction upon 

 impedance is a problem which arises in many ways. 



Choking coils for arc lamps are frequently made with a 

 movable core so that the self-induction can be varied, and 

 the impedance thereby increased or. decreased. In an alter- 

 nator the armature coils have a variable amount of self- 

 induction depending upon their position relative to the 

 magnet poles. In induction motors cases of a similar nature 

 are of frequent occurrence. . 



The self-induction also depends on the degree of saturation 

 of any iron forming part of the magnetic circuit, since this 

 affects its permeability. The current in a coil thus frequently 

 alters its self-induction. 



The following experiment is consequently of very great 

 practical importance, although the full calculations based upon 

 it will not be given until later. 



EXPERIMENT V. DETERMINATION OF DEPENDENCE OF 

 IMPEDANCE UPON SELF-INDUCTION. 



(1) Due to alteration of magnetic circuit. 



DIAGRAM OF CONNECTIONS. 

 As for Experiment IV. page 39. 



