164 Prof. E. Taylor Jones on the Potential 



coupling is 0-199 x h+J±* = 0-92. This is the coupling 



when the secondary terminals are connected with the 

 oil condenser. The mutual inductance L 21 is therefore- 

 0*49 henry. 



The same experiment was repeated with the primary circuit 

 closed through the small auxiliary coil (0*00099 henry) used 

 in the determination of the curve of fig. 5. The result in 

 this case was P = 0*80. Now from fig. 5 we concluded that 

 the coupling in the experiment in which that curve was 

 determined was 0*7. Hence the removal of the oil con- 

 denser from the secondary circuit, by rendering the secondary 

 current less uniformly distributed, reduces the coupling from 

 0*8 to 0*7, and we may assume that the same proportional- 

 reduction will take place whether the auxiliary coil be 

 present in the primary or not, since the presence of this coil 

 cannot affect the coefficients L 2l , L 12 , or L 2 . 



Thus without series inductance in either circuit, and 

 without a secondary condenser, the coupling of the magneto 

 circuits is 



P = 0-92x^=0-80. 



The effective resistances of the primary and secondary 

 circuits were determined from the logarithmic decrements 

 and periods of the oscillograph curves, with due allowance 

 in each case for the resistance of the auxiliary coil, which 

 was determined independently for about the same frequency.. 

 It was found that the effective resistances of the magneto 

 circuits for frequency 600 were very much greater than the 

 steady-current values. Thus the resistance of the primary 

 coil for steady currents was 0*85 ohm, and its effective 

 resistance at frequency 600 was found to be 49 ohms. The 

 secondary coil gave for steady currents 2115 ohms, for the 

 oscillations 42,670 ohms. These very great differences 

 arise mainly from core losses occurring during the oscilla- 

 tions, and are not found in air-core coils. For example, 

 the large air-core coil had a steady-current resistance 

 of 14,000 ohms, and an effective resistance at frequency 600 

 of 15,300 ohms. 



Specimens of the photographic curves used in these 

 measurements are shown in Plate VII. figs. 7 and 8 *. In 

 these cases the oscillations were started by the sparking 



* Full details as to the manner in which the frequencies are deter- 

 mined from the photographs have been given in a former paper (Phih, 

 Mag. Aug. 1907, p.' 242). 



