160 Discharge through a Coil of Variable Inductance. 



The chief difficulty that arises is in the drawing of the 

 earlier part of the curve, when the current is just starting, 

 and to do this accurately a supplementary curve is first drawn,, 

 a very much larger scale of current being employed. 



A curve of the same character may be drawn for the end 

 of the first half wave when the current has fallen to 20 

 amperes. The shape of the curve is, however, not exactly 

 similar on account of hysteresis. The actual case taken was 

 that of a coil on which a certain number of experiments had 

 been made. The coil was in the shape of a square with 

 about 90 turns of No. 18 g.p. wire on each limb. The self- 

 induction of this coil was measured without the core, and 

 found to be 4*1 x 10 ~ 5 henry. A core of soft-iron wire was 

 used in the experiment, and from the known permeability of 

 the soft iron for different magnetizing forces the curves shown 

 at A A were calculated. 



A series of curves has been drawn showing the shape of 

 the current- wave through the coil when the discharge from a 

 condenser of 1*25 micro-farads capacity flows, the condenser 

 being charged with (1) 9400 P.D. (2) 2350 P.D. (3) 450 

 volts P.D. It is interesting to notice the complete difference 

 in shape of the discharge curves. In the first case, that in 

 which the P.D. is 9400 volts, the current increases very slowly 

 until saturation is attained, that is, when the magnetizing^ 

 force is about 300, after which the discharge curve becomes 

 nearly sinusoidal. For the smallest voltage saturation is 

 hardly reached, so that the curve becomes peaked ; the effects 

 of hysteresis in this curve are clearly shown. The method is 

 of course adaptable to any kind of discharge, the curves in 

 question having been plotted to illustrate its working, and 

 because experimental values of the actual time occupied by 

 the discharge have been obtained. It will be noticed that 

 the agreement between graphical and experimental results is 

 fairly good. Exact agreement would hardly be expected on 

 account of the disturbing effects due to eddy- currents, which 

 are very powerful (at this frequency) with the finest 

 laminations. 



It is instructive to notice the increase in the time for a 

 half-oscillation when the P.D. before discharge is decreased. 

 The table below is taken for the voltages considered. 



'.D. before Discharge. 



Time for 1st Half-Oscillation. 



9400 



2-9 XlO" 5 



2350 



5 X 10- 5 



450 



15-2 x 10- 5 



It is clear, therefore, that with a discharge such as we are 



