Resistance, and Self- Cap achy of Magneto Windings. 37 



additional air-gaps introduced. In the rotating armature 

 magneto the maximum inductance also occurs before 

 advance break, for in this position the polarization and 

 reluctance of the armature magnetic circuit have minimum 

 values = 



Owing to diminution in the value of /jb a at high frequencies, 

 it follows from above that the inductance would not be ex- 

 pected to vary so much with, the angular position of the 

 armature. This also is corroborated by experiment. 



When the magneto is in operation and connected to a 

 peak-voltage measuring apparatus, the damping in the 

 secondary circuit is very large owing to the high effective 

 resistance of the iron under oscillations of audio frequency. 

 The effective inductance and effective resistance in this case 

 must be viewed in a different manner from that when the 

 oscillations are undamped, as will be shown later. 



The measurements described hereafter have been- carried 

 out with undamped oscillations as accurately as experi- 

 mental conditions would permit. None of the methods 

 was susceptible of a high degree of accuracy. Data were 

 required which showed the " order of magnitude " of dif- 

 ferent quantities and which exhibited the general behaviour 

 of the magneto under certain experimental conditions. These 

 conditions are different from those which obtain in practice. 

 The chief object of the research was to obtain some know- 

 ledge regarding the loss due to iron, etc. Using the values 

 of the coefficients of the circuits thus obtained, it is possible 

 to get an estimate of the efficiency of the magneto. A 

 B.T-H. polar inductor magneto was used in all experiments, 

 as it is well suited to research work. 



(2) Measurement of the Self-Inductance and Effective Re- 

 sistance of the Primary of a Magneto by Undamped 

 Oscillations. 



Before describing the method of measurement adopted 

 it is necessary to define the quantities it is intended to 

 obtain. If a sinusoidal current is passed through an 

 iron-cored coil, the voltage on the coil is nearly sinusoidal 

 provided the frequency is fairly high and the magnetizing 

 force low. At low frequencies, e. g. 50 ~ per sec, the voltage 

 wave form is not sinusoidal * owing to the variation in the 

 permeability of the iron with the current. The sinusoidal 

 voltage can be split up into two components R e I and coL e \ 



* See Journ. I. E. E. June 1915. 



