﻿Effective Adjustment of an Induction-coil. 7 



respectively. It will be seen that the two positive maxima 

 coincide at '0006 sec. after the interruption, giving the 

 maximum potential of 800,000 volts at the secondary 

 terminals. At this moment the potential of the primary 

 condenser is zero. Fig. 2 (C) shows (for comparison with 

 the oscillograph curves described below) the curve obtained 

 by squaring the ordinates of fig. 2 (B). 



The above theoretical conclusions apply strictly only to 

 the case of an ideal induction-coil, in which the resistances 

 of the circuits and other causes of damping of the oscilla- 

 tions are negligible. Some applications of the theory were* 

 however, given, in the former paper referred to *, in which 

 it was shown that the results of the theory agree closely 

 with experiment in so far as they concern the conditions 

 under which the greatest spark-length is given by an 

 induction-coil. The value of the secondary potential is of 

 course greatly modified by the damping. 



Another experiment will now be described, from the 

 results of which some further information may be gathered 

 as to the working of an induction-coil when in its most 

 effective adjustment. 



The coil experimented upon was the one employed in the 

 previous experiments, viz. : an 18-inch coil in which the 

 coefficient of coupling of the primary and secondary coils 

 (the secondary terminals being connected to the oscillo- 

 graph and to a variable spark-gap) is *876. The primary 

 circuit was fed by a number of storage-cells, and included 

 an amperemeter, a rheostat, and a slow interruptor which 

 broke the current about once per second. The condenser 

 was connected directly across the interruptor. 



The experiment consisted in increasing the self-inductance 

 of the primary circuit by the addition of air-core coils, and 

 varying the capacity of the condenser, until the longest 

 spark was produced by a given primary current. The curve 

 of secondary potential was then photographed, and the con- 

 stants of the circuits determined. The best effect was found 

 to be obtained with four extra coils in the primary circuit, 

 and with a condenser of *2 microfarad capacity. It was not 

 very easy to decide with great exactness upon the best self- 

 inductance and capacity, owing to the fact that the spark- 

 length varies slowly with these quantities in the neighbour- 

 hood of the maximum. A difference of *02f> mfd. either 

 way in C x caused no appreciable change in the spark-length. 

 From the practical point of view this slow variation near the 

 maximum is in itself no disadvantage, since very exact 

 * L. c. pp. 582 58(5. 



