40 Dr. McLachlan on Effective Inductance, Effective 



40 milliamperes. In finding the effective resistance it is ne- 

 cessaiy to vary the current. This causes a variation in 

 R c i at the lower frequencies. The value obtained for R el 

 increased with decrease in the current through the coii when 

 the circuit was thrown out of resonance — i. e., H el increased 

 with decrease, in I 2 for a given value of Xi- Owing to time 

 limitations, the inductances with currents larger than 40 milli- 

 amperes were not ascertained. The primary current in 

 a magneto is much greater than this (see Proc. Phys. Soc. 

 Dec. 1919). Since the peak-voltage is proportional to the 

 current broken in the primary from 0*25 amp. to 4 amp., it 

 is very probable that the inductances are constant throughout 

 this range. 



(3) Measurement of the Self -Inductance and Effective 

 Resistance of the Secondary by Undamped Oscillations. 



The methods of measurement adopted for the primary 

 winding are inapplicable to the secondary, owing to the 

 magnitudes of the quantities to be obtained. The effective 

 resistance of the secondary is of the order of 5 x 10 4 ohms at 

 a frequency of 1700 ~ per sec, and it is quite clear that 

 resonance could not be obtained on a milliammeter with this 

 resistance in circuit (2) of fig. 2 without the aid of extremely 

 large currents in circuit (1). The above quantities can be 

 tound, however, by modifying the resonating circuit in fig. 2. 



Fig. 3. — Diagrammatic arrangement of apparatus used to determine 

 the secondary inductance and effective resistance. 



Valve Generator 



r ->_• — 1 



nsromm 

 i 



© 



~y 



'f- 



Secondary 

 cf Magneto 

 in housing 



Variable 

 Condenser 



Electrostatic 

 Voltmeter 



The arrangement of the modified circuit is shown diagram- 

 matically in fig. 3. The magneto itself is used to obtain a 

 coupling between circuits (1) and (2). In this case the 

 aluminium end plates are removed and the primary is on 

 open circuit. The variable condenser is adjusted until the 

 voltmeter-reading is a maximum. Under this condition, 

 circuit (2) is in resonance. Knowing the frequency of the 



