TELEPHONE DIAPHRAGMS. 445 



with the diaphragm. It is assumed in this caHbration, that the dia- 

 phragm remains undefleeted from its position of rest through the 

 working range of luminous-spot deflection. With ordinary telephone 

 receiver diaphragms, it is found that the small pressures exerted by 

 the mirror produce no appreciable deflections, as these pressures are 

 less than 100 dynes, and the curves in Figure 9, show that such pres- 

 sures have inappreciable deflecting influences. 



In order that the exploring mirror M shall not break out of contact 

 with the vibrating surface of the diaphragm, to which it is applied, 

 it is necessary and sufficient that (1) statically, when the diaphragm 

 is at its maximum downward displacement, the point of the mirror 

 shall at least be in contact therewith, and (2) dynamically, that the 

 natural vibration frequency of the mirror shall be higher than that 

 of the diaphragm's vibrations under test, at resonance. This means 

 that it must ordinarily have a natural frequency of from 1000 to 1500 

 cycles per second. No difficulty has been found in accomplishing this 

 result. The phosphor-bronze strip does not need to be stretched very 

 tightly. 



Technique of Procedure for Determining Constants by Method 



OF Amplitude. 



The full procedure for determining the constants a, tn, r and *• of 

 a telephone receiver, uSing the method of amplitude measurement, 

 which the authors have found to be advantageous, is as follows : 



Connect the receiver to be tested in a Rayleigh bridge, Figure 12, for 

 measuring simultaneously its apparent resistance and inductance, 

 at various impressed frequencies, with a known r. m. s. testing-current 

 strength. A Vreeland oscillator ^^ is used as the source of sinusoidal 

 alternating currents, and by means of the resistance R, the voltage at 

 the terminals a^ and d is maintained at a constant value l)y electro- 

 static voltmeter VM. An anti-inductive resistance r\ sufficiently large 

 to keep the current in the bridge constant ^* is inserted between a and 

 A^ as shown. The two arms of the bridge ab and AC have equal 



13 Bibliography, No. 9. 



14 The method of testing at constant current is theoretically an improve- 

 ment over that of testing at constant voltage, and is due to the work of the 

 Western Electric Co.'s Engineering Department. The difference between 

 results at constant bridge voltage and constant bridge current, are, however, 

 ordinarily trivial. 



