64 • Dr. McLachlan on Effective Inductance, Effective 



This can be done in the following way. The initial primary 

 energy can be conveniently expressed in the form ^LF, 

 where L is some factor, usually termed the self-inductance, 

 and I is the current at break. It is difficult to give a 

 perfectly definite meaning to L owing to the peculiar con- 

 ditions which obtain in the magneto. The usual procedure 

 in measuring the value of L is to pass an alternating current 

 through the primary. The conditions which then obtain, 

 however, are very different from those just before the 

 primary circuit is broken. It is impossible to state whether 

 the value of L found by means of an alternating current is 

 equal to that in the expression -JLI 2 . L in this expression 

 would probably be found most accurately if the actual 

 energy in the primary for given values of the current and a 

 definite position of the timing lever could be ascertained. 

 In the absence of data with respect to L, we will take 

 the value as found by measurements with low frequency 

 alternating current. 



Here again there is a discrepancy since the inductance 

 varies with the current, so that we can only approximate 

 by taking the inductance of the armature at the point of 

 break for a current whose maximum value is equal to 

 the current broken in the primary. Knowing L x and I 1? at 

 break*, also the peak voltage, we can approach the energy 

 problem and determine the overall conversion efficiency from 

 primary to secondary, i. e., the ratio ^C 2 vV/iljeil., 2 - 



In the B-T.H. magneto used for these tests, L el = 6*5 

 x 10~ 3 henry at full advance break and I x = 2 amperes when 

 V 2 = 10 4 volts. Thus iL cl I 1 2 = 12-5 x 10" 3 joule. From the 

 calculation in the preceding section we have 



Energy lost in primary and secondary prior to sparking 

 + energy in spark 



= 3-1 x 10" 3 + 2-8 x 10- 3 = 5-9 x 10" 3 joule. 



Hence the energy unaccounted for is 



(12-5-5-9) 10" 3 = Q'6 x 10" 3 joule. 



This is unconverted into electrostatic energy prior to the 

 passage of the spark. Part of it is transferred after 

 the spark commences, the remainder being dissipated in 

 the primary and secondary windings. Thus we may, fol- 

 lowing Morgan (loc. cit.), consider that the energy, apart 

 from that derived from rotation, is supplied from the 



* McLachlan, Proc. Phys. Soc. Dec. 1919. 



