446 



KENNELLY AND AFFEL. 



resistances, wound anti-inductively. The arm CD contains adjustably- 

 variable resistance and inductance, for balancing the tested receiver 

 in BD. A pair of head telephones in the bridge wire BC, enables the 

 balance to be obtained conveniently, at any frequency between 400 and 

 2500 cycles per second, the range employed. It has also been found 

 convenient to use in each of the bridge arms ab and ac, a resistance 

 of 200 ohms, when testing ordinary receivers of about 75 ohms d. c. 

 resistance, and for the purpose of the amplitude measurements, a 

 testing current of 2 milliamperes r. m. s. in the receiver; or 4 milli- 

 amperes in the bridge through r\ with say 10 volts across a^d, which 

 calls for about 2250 ohms in r^. A series of resistance and inductance 

 measurements of the undamped receiver t, are then made over the- 



Fig. 12. — Inductance Bridge Connections for Testing Telephones. 



whole range of frequency to be covered, the frec^uency supplied by the 

 Vreeland oscillator being increased by convenient successive steps, 

 through adjustment of the capacitance in the Vreeland primary cir- 

 cuit. Near the resonant frequency of the receiver, these steps have 

 to be increased in number, and diminished in size, in order to follow 

 the rapid variations in the telephone impedance, as the frequency 

 passes through resonance. The free impedance of the telephone 

 thus becomes known over the range of impressed frequency. 



The amplitude measurer is then applied to the telephone as already 

 described, and the maximum dcRible amplitude, on its scale, of the band 

 of light formed by the oscillation of the reflected beam is adjusted 

 for in, frequency. It is also found very convenient to record this 

 amplitude over the entire range of frequency measurements. Half" 



