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DR. W. S. TUCKER AND MR. E. T. PARIS ON 
be varied at will. This can easily be made by fitting a wooden plunger inside the 
brass tubing of the container, or by making the container in two parts of different sized 
tubing so that one part will slide over the other. 
Another method of tuning the microphone is to alter the length of the neck. It is, 
however, inadvisable to reduce the length of the neck to less than 1 cm., as with shorter 
necks than this the grid is exposed to the effect of transient currents of air. 
§ 3. Electrical Connections. 
When the air in the neck of the resonator is set in vibration by a sound of suitable 
frequency, the platinum wire grid suffers a change in resistance, which may be regarded 
as being made up of an oscillatory change and a steady change. There are thus two ways 
in which the microphone can be used. 
(i) The Amplifier Method .—If the oscillatory effect is to be observed it is necessary to 
include an amplifier in the circuit. A suitable form for the circuit to take is shown in fig. 3, 
where the microphone (M) is connected in series with a battery (B), a milliannneter (A), 
a rheostat (R), and the primary of the input transformer of a three-valve amplifier.* 
The sound can of course be heard in the telephones, and provided that the grid lies 
in an approximately horizontal plane (i.e., that the axis of the neck of the resonator 
is vertical) the pitch heard is the same as that of the original sound. The effects which 
occur when the grid is moved out of the horizontal plane are described in § 8 ; but it may 
be noted here that not only the pitch of the note heard in the telephones, but also the 
sensitivity of the microphone, depend on the inclination of the grid to the horizontal 
plane. It is therefore important in any experiment where comparisons of the strength 
of sound are being attempted, that the position of the microphone relatively to a 
horizontal plane should not be changed during the experiment. The difficulty can be 
overcome by arranging that the microphone shall always hang so that the axis of the 
neck is vertical. Small deviations of one or two degrees from this position do not 
materially affect the sensitivity. 
* Several different types of amplifier have been tried for this purpose. The best results have been 
obtained with a four-valve resistance amplifier specially designed for low acoustic frequencies. 
