Year: 1891

page_text: Date of Application, 9th Dec., 1891-Accepted, 9th Jan. 1892
Figure 5 is a section of the complete microphone.
The electrodek which is made of conducting material is provided, on the side
which faces the diaphragm m, with pyramidal projections, each of which has on its
apex a small brush p, Figure 2, of insulating elastic material such as wool, cotton,
silk, felt, hair or the like.
The pyramidal projections on the electrode k alternate, that is to say, the
junctions of the bases of one row are situated in the middle of the sides of the bases
of the next row, as clearly seen in Figure 1.
The object is to prevent small grains of carbon c, Figure 5, from falling down,
that is to say, when the microphone is in a vertical position, the grains can only
fall from one pyramidal surface onto the next lower one, instead of falling right
down through a continuous groove; by this means the grains of carbon are
prevented from collecting together and impairing the sensitiveness of the
diaphragm.
The diaphragm m is covered nearly to the centre with a thin layer of elastic
insulating fibre, Figures 3 and 4, such as wool, cotton, silk or other elastic
insulating material. As shewn in Figure 5 the grains of carbon c are in contact
only with the inner side of the central part of the membrane m and with the whole
face of the electrode k.
The result of this arrangement is to render the diaphragm m extremely sensitive
to sound waves as the elastic insulating fibres g are placed opposite the brushes p
of the electrode k and only the central part of the diaphragm m is in conducting
contact with the grains of carbon c, that is to say, that part of the diaphragm
where the strongest vibrations occur.
The electrode k is fixed by means of a screw 8 in a case 6 of insulating
material, Figure 5, the screw at the same time holding the connecting wire
leading to the contact screw.
The case is placed in an outer metal casing i and fixed with screws. After
having filled the recesses of the electrode k with granulated carbon c, the dia-
phragm m is laid on it with its side g in contact with the carbon, and the
cover with the mouth-piece t and felt ring f is screwed on. A wire connects
the diaphragm m to the positive contact screw.
The microphone, as shewn in Figure 5, is always used in a vertical position
which causes the carbon grains on the upper inclined surface of the pyramidal
projection to press gently against the diaphragm m but the said grains are only
allowed to come into conducting contact with the diaphragm at its central part
Price 6d1

page_text: as the rest of its surface, Figure 3, is provided with the thin layer consisting of
particles of wool, cotton, felt, hair or like insulating and elastic substance.
The result of this arrangement is that the carbon grains care as much as
possible secured in their position and the contact alterations, which are so trouble-
some in all other microphones, are reduced to a minimum.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed, I declare that what I
claim is :-
1. In a microphone the use of an electrode having its surface formed with
pyramidal projections alternating in the manner described for the purpose of
preventing the carbon grains from falling.
2. In a microphone the use of a vibrating diaphragm made of conducting
material such as carbon and provided on one side, except on the central part, with
a thin layer of insulating elastic fibre such as wool, cotton, silk and the like, sub-
stantially as herein before described.
3. The improved microphone herein before described and illustrated in the
accompanying drawing.
Dated the 9th day of December 1891.
G. F. REDFERN & Co.,
4, South Street, Finsbury, London, Agents for the Applicant.