IMPEDANCE. 



39 



A curve obtained in this way is shown in Fig. 17. The 

 readings here plotted were taken on a 10-ampere arc lamp 

 choking coil having a laminated core and cast-iron base, 

 completing the magnetic circuit. The resistance of the coil 

 was -15 ohm, and the frequency of the current 45 cycles. 



It is to be noticed that the curve bends sharply at first, but 

 becomes practically a straight line after a short distance. 

 This is owing to the relatively great importance of the 

 reactance compared with the resistance at first, where the 

 resistance is small, so that the curve at first is nearly horizontal, 

 since the resistance has hardly any effect in increasing the 

 impedance. As the resistance becomes considerably greater 

 than the reactance, the impedance increases more rapidly 

 for a given increase of resistance, and finally increases 

 practically in the same proportion as the resistance, as shown 

 by the line becoming straight. 



From the curve it is seen that the value of the impedance 

 when the resistance is zero is 5-2 apparent ohms. This is 

 consequently the value of the reactance of the coil = 2 irn L. 

 From this value L, the coefficient of self-induction of the 

 coil could be calculated from the known frequency. 



The upper part of the curve approaches, and would 

 ultimately become tangential to a straight line drawn through 

 zero at an angle of 45, i.e., passing through the points 

 having ordinates and abscissae equal. 



The next experiment shows that the impedance of a given 

 circuit or coil does not depend only on the coil or resistance 

 itself, but on the frequency of the current sent through it. 



EXPERIMENT IV. DEPENDENCE or IMPEDANCE UPON 

 FREQUENCY. 



DIAGRAM OF CONNECTIONS. 



FIG. 18. 



