Capacity for Induction-coils and Tesla Coils. 229 



at the 7/1 coincidence. As u diminishes from this value 

 U sin </> goes through a series of maxima and minima, 

 the maxima corresponding approximately to the ratios 

 n 2 /7i 1 =ll, 15, 19, . . . There is also another maximum o£ 

 Usin<£ at about w = 0*46 {n 2 jn l = 4*237), and a marked 

 minimum near u = 0'25 (n 2 /^! = 5). 



Fig. 1. 



,2 



0-9 



0-8 



0-4 



0-6 



0-8 



Curve showing the variation of maximum secondary potential of an 

 induction-coil when the primary capacity is varied, k 2 being 0768. 

 The abscissa represents u ( = L 1 C 1 /L 2 C 2 ), the ordinate of the full- 

 line curve TJ sin <p } of the broken-line curve U. 



Some of the chief features of the U sin $ curve may be 

 traced experimentally by observing the spark-lengths ob- 

 tained with various primary capacities. Thus, with the 

 18-inch coil, the greatest spark-length is found to occur 

 near u = 0'l (0i = O06 mfd.*), there is a minimum spark- 

 length when u is about 0*25 (Ci = 0'15 mfd.), and a second 



* These spark-length measurements were described in a former paper 

 (Phil. Mag. April 1914, pp. 582, 583), in which O'Oo mfd. was given as 

 the optimum capacity. At that time I was not able to try O06 mfd., 

 but I have since found that this represents more accurately the optimum. 



