A SELECTIVE HOT-WIRE MICROPHONE. 
40-°, 
in vibration at a convenient distance from the microphone. The amplitude of vibration 
of the tuning-fork could be observed through a microscope, and it was found that, with 
care, its amplitude could be maintained at a given value to within 2 or 3 per cent. A 
second series of observations of current and resistance of microphone grid was then made 
and the new current-resistance curve plotted on the same chart. The results obtained 
in a particular experiment are given in the following table:— 
Without 
1 
Sound. 
With Sound. 
Current in 
Resistance 
Current in 
Resistance 
milliamperes. 
in ohms. 
milliamperes. 
in ohms. 
10-0 
157-5 
11-0 
157 • 5 
13-6 
177-5 
14-9 
177-5 
16-1 
197-5 
17-9 
197-5 
18-5 
217-5 
20-3 
217-5 
20-3 
237 • 5 
22-6 
237-5 
22-1 
257-5 
24-5 
257-5 
23-8 
277-5 
26-3 
277-5 
25-2 
297-5 
28-0 
297-5 
26-7 
317-5 
29-5 
317-5 
28-0 
337-5 
The current-resistance curves are plotted in fig. 7. If, for some particular value of 
Fig. 7. 
the heating current, the ordinate of the lower curve is subtracted from that of the upper 
curve, we find the change dR which the sound produces in the resistance of the grid 
