402 
DR. W. S. TUCKER AND MR. E. T. PARIS ON 
in what manner the microphone is employed, it is found that its sensitivity is always 
increased by increasing the working current. The curves in fig. 6 show this effect in 
Fig. 6. 
two cases : (1) when the microphone is connected in series with the primary of a trans¬ 
former, the secondary of which is joined to a vibration-galvanometer ; and (2) when 
the microphone is employed with an amplifier and vibration-galvanometer. 
The curves were obtained by clamping the microphone so that its orifice lay just 
between the prongs of an electrically maintained tuning-fork making 250 vibrations 
per second. The tuning-fork was carefully maintained at a constant amplitude while 
the heating current of the microphone was gradually increased from zero to the maximum 
safe current of about 28 -5 milliamperes. In fig. 6 the heating current is plotted against 
the deflection of the vibration-galvanometer. Curve I refers to the case when the micro¬ 
phone is used with a transformer alone, while the effect of introducing an amplifier is 
shown by Curve II.* In the latter case, however, it must be borne in mind that the 
amplification itself is in all probability a function of the magnitude of the effect produced 
in the microphone. It will be observed that the effect produced by the sound is almost 
negligible until the heating current reaches a value of nearly one-third of the safe 
maximum. 
In the case of the Wheatstone’s Bridge, the effect of a change in the heating current 
of the microphone is complicated by the altered sensitivity of the Bridge. The variation 
of the sensitivity was therefore investigated by measuring the change in resistance of 
the grid with various heating currents for a given constant value of sound intensity. 
The method adopted in the experiment was as follows. A microphone was connected 
into a Wheatstone’s Bridge circuit in the usual manner, and its resistance measured with 
various heating currents so that a current-resistance curve could be plotted. An 
electrically maintained tuning-fork, with a resonator to reinforce the sound, was then set 
* For convenience the sensitivity of the gafvanometer was reduced when using the amplifier. 
