482 Dr. Norman Campbell on the "Break " 



that, in the absence of a primary condenser, the maximum 

 secondary potential could be very considerably increased by 

 increasing sufficiently the speed of the break. This conclusion 

 was confirmed by Prof. Taylor Jones; but the further 

 conclusion that, if the speed of the break were increased 

 sufficiently, as great a secondary potential could be obtained 

 without a condenser as with one, was not confirmed. Theory 

 and experiment alike show that, even if the break is perfect 

 and there is no sparking, there is an optimum value for the 

 primary capacity, depending on the other constants of the' 

 inductive circuits ; and that the use of a capacity either 

 greater or less than this optimum value involves some 

 sacrifice of efficiency. 



Accordingly, in order that the greatest possible efficiency 

 should be obtained, it is necessary that this optimum 

 primary capacity should not be less than that which is 

 necessary to prevent sparking ; for otherwise, if a capacity 

 is used large enough to prevent sparking, there will be a 

 loss of efficiency, small but important, due to the use of a 

 primary capacity greater than the optimum. In all magnetos 

 which have been examined, and probably in all induction- 

 coils with all forms of break, this necessary condition is not 

 fulfilled ; the optimum capacity is less than that required to 

 prevent sparking. Most magneto manufacturers are aware 

 that the secondary potential developed increases steadily as 

 the primary capacity is diminished up to the point where 

 sparking begins; the capacity of the primary condenser 

 which it is necessary to use is determined wholly by the 

 necessity for avoiding sparking. 



It becomes of importance, therefore, to inquire whether 

 the capacity necessary to avoid sparking might not be 

 diminished by changing some other characteristic of the 

 break. The only characteristic which seems to have been 

 considered in this connexion is the speed of separation of 

 the terminals. According to the view which seems generally 

 accepted, the spark between the separating terminals occurs 

 because the rate at which the potential difference between 

 those terminals increases as a result of the interruption of 

 the current is greater than the rate at which the spark 

 potential of the gap between the terminals increases with 

 the distance between them *. The primary condenser stops 



* This theory is formally stated and supported by some experiments 

 in a paper by H. Wiesinger, Ann. d. Phys. lv. 6, p. 401 (1918) ; but it 

 must have occurred to everyone who has paid nny attention to the matter. 

 It will be seen later that the conditions of the experiments by which 

 Wiesinger supported his theory were not those obtaining in the primary 

 circuit of a magneto or induction-coil. 



