Resistance, and Self- Capacity of Magneto Windings. 39 



mean o£ the tAVO values of the inductance is taken as 

 the inductance at resonance, and the variometer is set 

 accordingly. The magneto is then disconnected and the 

 inductance again varied until resonance is obtained. The 

 difference in the readings is the effective inductance of the 

 primary winding of the magneto. 



The effective resistance is found in the well-knou n manner 



by obtaining (a) the current at resonance, (6) the current 



when the circuit is out of resonance. The latter condition 



can be obtained by varying either the condenser or the 



inductance. In these experiments the latter, which had 



a constant resistance, was varied. The difference in current 



should be appreciable to obtain accurate results.. It is 



important that the current in the primary oscillating circuit 



should be the same for cases (a) and (b). If it varies, a 



correction can be made, provided the two values are known. 



Let I x = current at resonance with magneto in circuit. 



I 2 = current when circuit is out of resonance. 



L = variation in inductance to put circuit out of 



resonance. 

 a) = pulsatance = 2irf. 

 H el = effective resistance of primary winding at fre- 

 quency /'. 

 r e — effective resistance of circuit without primary of 

 magneto. 

 Then 



^+B el = "Htt^tJ^ ft,L ( ( i ] /i 2 ) 2 _i) 2 - • & 



r e is found in a similar manner to that of r e + ^{ e \. 



A more accurate method, if the apparatus is available, 

 is to employ a bridge as used by Hund *. In this way it is 

 possible to find the inductance and resistance for the funda- 

 mental and various harmonics. This method should be 

 useful where currents of the same magnitude as the maximum 

 primary oscillatory current under working conditions are 

 employed, since in this case there would be harmonic- 

 owing to the apparent permeability of the iron not being- 

 constant over the range of values of H employed. 



In the present experiments, it was found that the inductance 

 and effective resistance varied slightly with the current at 

 the lower frequencies when the armature was situated in the 

 housing. Throughout the experiments, the root mean 

 square current used in measuring the inductance was about 



* Electrician, vol. lxxv. p. 78 (1915). 



