the Induction- Coil. 715 



adopted. The condenser having been disconnected from the 

 primary circuit, a current was passed through the primary 

 coil and the curve of secondary potential, produced at its 

 interruption, was photographed. Under these circumstances 

 the oscillations of the system depend only on the constants of 

 the secondary circuit, and should give the value of the 

 product of the self-inductance and capacity of the secondary 

 coil. The photograph showed, in fact, a single damped 

 oscillation with period' lengthening as the oscillations died 

 away owing to the variation in the permeability of the core, 

 The period was measured in the early part of the curve, 

 where the amplitude is large, and was found to be '0017G3 sec. 

 Taking this as being, equal to 27r^/L 2 C 2 , we have 



L 2 C 2 = 0787.10- 7 . 



A condenser, of small known capacity C 2 ', was then con- 

 nected to the secondary coil and the period of this circuit, 

 with the primary open, determined. This gives in the same 

 way the value of L 2 (C 2 + C 2 / ). From these two results the 

 values of L 2 and C 2 can be determined. The values so found 

 were L 2 = 2350 henries, C 2 = 0000335 microfarad. These 

 values were used in working out the expression (1) for the 

 following three cases. It may be noticed, however, that it 

 is not so important to know the values of L 2 and C 2 separately 

 as it is to know their product ; C 2 occurs separately only in 

 the damping factors of the two oscillations represented 

 by (1). 



The se!f-inductance of the primary coil was determined 

 by connecting to it the largest capacity available (20" 73 

 microfarads), and photographing the curve of secondary 

 potential produced at the interruption of a current in the 

 primary. The curve obtained shows a double oscillation ; 

 but, since LjCx is much greater than L 2 C 2 , one of the oscil- 

 lations has a much longer period than the other, and it is not 

 difficult to determine the frequency of the slower oscillation 

 from the photograph. On substituting in the expression 

 (2) we obtain the value of L x . 



As the potentials obtained in the following experiments 

 were higher than those in the former cases, the oscillograph 

 was made less sensitive by withdrawing the attracting plate 

 to a greater distance from the steel strip. To prevent sparking 

 inside the instrument the attracting plate was enclosed in a 

 thicker sheath of ebonite. The distance of the edge of the 

 plate from the strip was about 7 mm. 



