A SELECTIVE HOT-WIRE MICROPHONE. 
393 
Ej made of the same material. Ej is provided with the terminal T r The mica plate (M) 
carrying the grid is clamped between Ej and the lower ring E 2 , which is also of brass 
and carries the terminal T a at the side of the holder. Beneath E 2 is a rubber ring Iv r 
and this rests on a bed of ebonite (P) to which also the plate E x is fixed by the screws S. 
The ebonite bed (P) is square, and is bolted at the corners to the square brass plate (B) 
which forms one end of the container. To ensure an air-tight joint a square plate of 
thin rubber R 2 is inserted between the holder and the container. 
When the plate E L is screwed down on to the ebonite bed, so that the mica plate with 
its silver foil electrodes is firmly held between E : and E 2 , a current can be passed through 
the grid by connecting a battery to the terminals T, and T 2 . 
The neck (A) forms the channel of communication between the interior of the 
container and the outside air, and from an acoustical point of view is the most important- 
part of the holder. If the capacity of the container be given, it is on the hydro- 
dynamical conductivity of this neck that the pitch of the resonator depends. The 
dimensions of the neck generally used were : length 2 -2 cms., internal diameter 0 -75 cm. 
In certain experiments, however, the neck was made rather shorter than this in order 
to tune the resonator to some given pitch. 
When the grid is being placed in position between E : and.E 2 it is important to see 
that the circular aperture in the mica plate is coaxial with the neck, since even a small 
displacement from this position mil change the pitch of the resonator by an appreciable 
amount. 
(iii) The “ Container .”—The containers were in most cases made from brass tubing. 
One end of the tubing is closed with a circular brass plate, while at the other end is fitted 
a square brass plate of the same dimensions as the base of the ebonite bed of the holder, 
which is bolted to it by means of the bolts b, as shown in fig. 2. A circular hoie |-inch 
in diameter is cut in the middle of this square plate to allow a free passage of air through 
the neck into the container. The thickness of the brass of which the tubing was made 
was 1 mm. 
The natural pitch of the resonator of course depends on the volume of the container. 
Thus, with the form of neck described above, a volume of 290 c.c. gives the resonator 
a pitch of 116 vibrations per second, while a volume of 68 c.c. gives it a pitch of 240 
vibrations per second. For pitches below 200 vibrations per second it has been found 
convenient to use brass tubing from 2 to 2| inches in diameter, while for higher pitches 
(above 200) tubing about 1 inch in diameter is the most suitable. 
Other forms of container have been made and tested, and reference to some of these 
will be found in a later paragraph. The material from which the container is made, 
and the thickness and rigidity of the walls, have a very marked effect on its resonating 
properties. The most efficient resonator which was tested was one which had been 
drilled out of a solid piece of brass, and its superiority must be attributed, in the main, 
to the increased strength of the walls. 
For experimental purposes it is often desirable to have a microphone whose pitch can 
