ivhen the Frequency is below a certain Critical Value. 405 

 is to shift the point A' corresponding to the maximum value 

 of j- from midway between and B' and towards B'. 



The author was enabled, through the kindness of Dr. 

 Fleming, to try an experiment in the meter-testing room of 

 the Electric Supply Company, using a Kelvin balance for 

 measuring the currents through the primary coil. The alter- 

 nating P.D. was obtained from the terminals of a transformer 

 capable of giving over 100 A ; and as the maximum current 

 ever taken was 6^ A about, the P.D. may be assumed constant. 

 The frequency was 83*3. 



The air-core transformer used for the experiment consisted 

 of two coils wound one inside the other, and of No. 20 B.W.G. 

 cotton-covered wire. The inner coil was used as primary 

 and the outer as secondary. 



Each coil consisted of 5 layers, of 125 turns per layer. 



Calculating the time-constants or values of ^ for the coils 



by Perry's approximate formula, they worked out as '00121 

 for the inner coil or primary, and '00152 for the outer coil or 

 secondary 



£> = 2ttx 833 = 523. 

 Hence 



ai =-00121x523 = -633, 



a 2 = -00152x523 = -795. 



The values of a x and a 2 were probably smaller than these 

 values, as the coils became fairly w T arm from working ; more- 

 over the primary had the leads and the resistance of a Kelvin 

 balance in series with it. 



Take, then, a ± as *5, and ct 2 as '7. 



The observed value of j- was 1*032. 



Substituting these values in equation (13) makes /3 = '57, 

 or a magnetic leakage of 43 per cent., if the P.D. were a true 

 sine function. This seems a rather large value for the leak- 

 age ; and it is probable that the leakage was considerably less 

 than this, but that the P.D. was not a pure sine function, and 



on this account the ratio j- was less than it otherwise would 

 have been. [ 



The above experiment was only a rough one, but it showed 

 an increase of 3*2 per cent, in the impedance of the primary 

 on closing the secondary : moreover the time-constants of the 

 coils were not suited for giving the best effect with the 



Phil. Mag. S. 5. Yol. 37. No. 227. April 1894. 2 E 



