24 On Metal Microphone* in vacuo. 
microphone is sealed by means of electrodes of stout pla- 
tinum wire, g, g, g' . The two wires g, g have a plate or strip 
of iron -wire gauze, of about 80 meshes to the lineal inch, 
stretched tightly between them. This gauze forms the fixed 
contact of the microphone. The movable contact is formed of 
a smaller square of similar gauze hung or hinged on the third 
electrode, </,in such a manner that it rests lightly against the 
fixed piece behind. This metal piece is so light that it forms 
a microphonic contact with the fixed piece below ; and when 
a circuit is made by a battery (B) and a telephone (T) through 
the gauze, the ticking of a watch, ic, laid on the base- board is 
heard in the telephone. 
The sensitiveness of the microphone is increased by bring- 
ing up the pole of a magnet in front of the movable piece so 
as to lighten the pressure of that piece on the fixed plate 
behind, and tend to draw, by the inductive attraction of the 
pole upon the iron, the movable piece away from the piece on 
which it rests. I have not been able to ascertain whether the 
movable piece is actually separated from the back piece by 
the approaching magnet, because in the case of metals, when 
the microphonic contacts are separated by an extremely minute 
space, the current is interrupted, whereas in the case of a car- 
bon microphone Mr. Stroh has observed an actual separa- 
tion of the contact-points by means of a microscope, whilst 
the current continued to flow and the microphone to act. It 
is certain, however, that as the pole of the magnet is gradually 
brought nearer to the movable gauze the ticking of the watch 
becomes gradually louder and better defined, as if the contact- 
points were able to vibrate in a freer manner and with a longer 
range. At last there comes a point at which the sonorous 
current, which had evidently increased till then, suddenly 
fails, and a sharp click in the telephone announces the fact that 
the electric flow is interrupted. 
If, on the other hand, the pole of the magnet be brought up 
behind the fixed gauze, the sound in the telephone dies off, 
apparently because the movable gauze is pulled by its attrac- 
tion into closer contact with the fixed piece behind. 
On exhausting this bulb with a mercury-pump of the im- 
proved kind used for making electric incandescence lamps, 
the ticking of the watch becomes singularly clear and metallic 
in quality, the very ring of the hair-spring seeming to be 
audible. In fact a vacuum microphone of this kind is pecu- 
liarly sensitive; and the effect appears to be due, in part at 
least, to the rarity of the air within the bulb. Another form 
of the instrument, having loose iron grains contained in a 
vacuum-bulb between two electrodes of platinum, exhibits the 
