KENNFXLY AND PIERCE. — TELEPHONE RECEIVERS. 149 



Voltage about 20 ohms = 0.129 volt 



" about secondary =0.125 " 



" about both =0.190 " 



" about both with secondary reversed = O.lfU " 



Substitution of the first three of these values in the formula for an 

 obtuse-angle oblique triangle gives 79°, as the angle by which the 

 secondary voltage leads the primary current. This is the angle 

 by which the time derivative of the magnetic flux leads the primary 

 current. The flux itself lags its time derivative by 90°, and therefore 

 lags the primary current by 90°— 79° = 11°. 



Again, a substitution of the first, second, and fourth vakie of above 

 table in the formula for an acute-angle oblique triangle gives for the 

 flux lag angle the value 11.5°. 



This angle of lag of flux behind the magnetizing current was found 

 to be nearly independent of the frequency. To illustrate this, and as 

 a further confirmation of the result obtained by the three-voltmeter 

 method with the crystal rectifier and galvanometer as voltmeter, 

 a second measurement was made by an entirely different method; 

 namely, by the use of a Drysdale alternating-current potentiometer, 

 with a 60-cycle current, and with a vibration galvanometer as indi- 

 cating instrument. The method employed in this experiment con- 

 sisted in first measuring the magnitude and phase of the primary 

 current, and then the magnitude and phase of the voltage in the 

 secondary winding. The difference between these two phases, sub- 

 tracted from 90^ gives the required angle of magnetic flux lag. Bal- 

 ance was in each case indicated by getting a zero deflection of the 

 vibration galvanometer. This method gave 12.5° as the angle by 

 which the flux in the telephone lags behind the magnetising current. 



The three values obtained by direct measurement for the flux lag, 

 which should be the angle /? according to the theory above proposed, 

 are 11°, 11.5°, and 12.5°; whereas half the depression angle, for this 

 telephone, which, according to the theory, should also be the angle j9, 

 is 13.2°. The agreement is not as good as might be desired for a 

 perfect conflrmation of the proposed theory; but in view of the diffi- 

 culty of measuring small angles of lag in circuits containing voltages 

 of the order of 0.1 volt, and in view of the fact that the experimental 

 telephone receiver constructed for this purpose had to be complicated 

 by auxiliary secondary windings and also unfortunately had a dia- 

 phragm mounted in such a way as to have a very large temperature 

 coefficient of vibration period, which rendered difficult an accurate 



