the Induction- Coil. 709 



The interrupter consisted of a copper rod making contact 

 with the interior of an iron vessel containing petroleum. 

 The rod was raised automatically, as the reflected rays of 

 light were crossing the photographic plate, by a lever placed 

 beneath the rotating mirror. 



Experiments with a Condenser in the Secondary Circuit. 



When a condenser is connected to the terminals of the 

 secondary, the capacity of the secondary coil only comes in 

 as a small correction, and it is unnecessary to know its value 

 very accurately. It consists partly of the internal capacity, 

 depending upon the capacity of one section of the coil on the 

 next and upon the number of sections, and partly of the 

 external capacity which depends upon the position of 

 the coil with respect to other bodies and especially with 

 respect to the primary coil. The methods by which these 

 capacities were determined were described in a previous 

 communication *. 



Case I. — In the first experiment with the smaller in- 

 duction-coil the primary was drawn out of the secondary 

 until the curve of secondary potential, as shown by the 

 oscillograph, had a simple and well-marked form, cor- 

 responding probably to some simple ratio between the two 

 frequencies of oscillation. This case was intended as a 

 preliminary test of the method. 



The photograph is shown in Plate XII. fig. 8. The current 

 broken in the primary circuit was 5*05 amperes, and the 

 ordinate of the curve represents the square of the secondary 

 potential at any subsequent time. The curve was photographed 

 for four different values of the primary current i ranging 

 from 2*8 to 5*05 amperes, and the general form of the curve 

 was the same in all, only the amplitude varying with the 

 current. The amplitude of the first wave in each was 

 measured on the negative with a travelling microscope and 

 the values found were, within 4 per cent., proportional to 

 the squares of the primary currents. 



The lower curve in the photographs represents the oscil- 

 lations of a 768 tuning-fork photographed simultaneously. 

 Immediately after these curves were obtained another curve 

 was photographed showing the secondary potential for an 

 air-core induction-coil with known capacities and inductances 

 in its circuits. This curve was similar to one of those 

 discussed in the previous paper above referred to, and to 

 which the expression (1) may be assumed to apply. By 



* E. T. J ones, Phil. Mag., August 1907, p. 248. 



