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BELL SYSTEM TECHNICAL JOURNAL 



fully (lone and the impedance bridge measured impedance accurately 

 with currents as low as IQ-^ amperes, through the receiver under 

 test. The correctness of the point of balance of the bridge was estab- 

 lished by measurements of standard impedances over the range of 

 currents employed in the receiver tests. A schematic diagram of 

 the circuit is shown in Fig. 1. 



For measurements of damped impedance, the receiver was placed 

 in a small sound-proof box, with its diaphragm damped by a microm- 

 eter depth gauge, which was carefully adjusted so as just to impinge 

 upon the diaphragm. It was necessary to insulate the receiver from 

 mechanical agitation, since minute voltages generated in it were 

 sufficiently amplified to cause an excessive noise in the head receivers. 



Fig. 2 shows the damped eflfective resistance and reactance of the 

 six instruments, taken at 1,000 cycles, plotted on semi-logarithmic 



Mk. 1 



paper. It will be seen lii.n Inluw approximately 10 '' amperes, the 

 impedance is constant. However, above this value both the efTective 

 resistance and the reactance show a consistent increase with the 

 current. The minimum current employed (lO"" amperes), is between 

 two and three times the minimum audible current for this type of 

 instrunient, but from the data- taken there is no reason to suppose 

 that the impedance would vary for smaller currents. This receiver 

 has a winding of 11,000 turns, and it can, therefore, be assumed that 

 this type of structure will have constant impedance below a magneto- 



