Resistance, and Self- Capacity of Magneto Windings. 65 



primary in the electrostatic form np to the time of sparking 

 and thereafter in the inductive form as a current in the 

 secondary circuit. It is the former component which 

 determines the ignition of an explosive mixture. 

 The overall conversion efficiency of the magneto is 



280 99 f 



jar* = 2z per cent. 



Taking now the figures obtained by using the formulae 

 of Taylor-Jones, we find that the secondary energy, if there 

 were no loss, is 4'4x 10~ 3 joule, whereas the former method 

 gave 5 "9 x 10 ~ 3 . 



(J>) Effect of Variation in Inductance on Peak Voltage 

 and Energy Loss. 



Apart from the reduction in peak voltage due to dis- 

 sipation of energy in the primary and secondary windings, 

 there is also a reduction caused by the diminution in 



inductance. It has already been shown that u = T el n - 



is nearly constant. The value of k does not undergo an_y 

 very pronounced variation up to /'=6000^-; it decreases 

 with /. owing to the reduction in apparent permeability 

 of the iron, but the rate of decrease is not very large. Foi 

 simplicity k will be taken as constant. Moreover U sin cf> 

 is constant. Thus at full advance the ratio of the peak 

 voltage with unvarying inductance to that in the case cited 

 above where f— 6000 ~, is approximately equal to 



v/L el to steady currents 



VL el at /=6000 — 



1-44. 



-\P 



W4 



Hence the diminution in inductance causes a drop in the 

 theoretical peak voltage of 31 per cent., provided of course 

 the value of nJni due to increase in " k" does not increase 

 beyond 5 and so alter the phase relations of the two 

 oscillations which occur after break. 



Substituting the direct-current or low-frequency values 

 of the coefficients in (.10), we find that the peak voltage 

 is 18,200. In practice it is 10,0.00, i. < ; . 55 per cent. o( 

 tl^e ideal. 



Phil. Mag. S. 6. Vol. 41. No. 241. Jan. 1921. F 



