48 Dr. McLaehlan on Effective Inductance, Effective 



audible frequency, or a valve amplifier may be used in con- 

 junction with a rectifying valve, smoothing out condenser, and 

 a microammeter. Care must he taken, however, with regard 

 to self-resonance effects in the differential transformer and 

 the valve-amplifier circuit. The current in the bridge can 

 be measured by using a transformer and amplifying if the 

 current is small, or by a thermoammeter reading to 100 milli- 

 amperes, if the current is large enough. 



(6) Results of Experiments. 



(a) Armature in Housing. 



The values of the coefficients of the primary and secondary 

 circuits obtained with the armature in the housing are showm 

 in figs. 10 to 14. It will be seen that the inductances decrease 

 rapidly until the frequency is 15,000— per second. Beyond 

 that value the decrease in inductance is much more gradual, 

 owing to the fact that the effective permeability of the 

 magnetic circuit is approximately constant. This is in 

 agreement with the argument at the beginning of the paper. 

 The inductance, especially at low frequencies, depends on the 

 position of the rotor, being greatest just before advance break 

 and least after retard break. The difference in the values 

 after advance and after retard break was only a small 

 percentage. The same holds good with reference to the 

 effective resistance. This variation is due to the change in 

 the reluctance and polarization of the magnetic circuit with 

 the position of the rotor. As the frequency increases the 

 difference between the inductances and resistances for 

 various rotor positions decreases, owing to diminution in 

 the apparent permeability of the iron. The ratio of the 

 secondary to the primary inductance is shown in Table I., 

 and it will be seen tbat the variation with frequencv up to 

 5000^ is small. 



Table I. 

 Showing Ratio L e2 /L e i. Armature in housing ; rotor 

 position after full advance break. Values of L e2 

 obtained by calculation. 





/ 





Le2 Lfi 



— per 



see. 



with ikon cor 



17 



X 



10 3 



4-4 X 103 



3-5 





,, 



4-5 ,. 



5 





,, 



4'55 ,, 



10 





,, 



4-66 „ 



15 





,, 



4-76 , 



20 





„ 



4-85 „ 



35 





)> 



5-35 „ 



with air core. 



11-5 X 10 3 



0234 x 10' 



