Thermophone.
An apparatus for reproducing sounds telephonically by the agency of
heat; a receiving telephone actuated by heat. Thus a wire may be
attached to the centre of a diaphragm and kept in tension therefrom, and
the transmitting telephone current may be caused to pass through it. The
wire changes in temperature and consequently in length with the pulses
of current going through it and vibrates the diaphragm, reproducing the
sound. It is to be distinguished from the thermo-electric telephone
which involves the action of potential difference produced by
thermo-electric action.
Thermostat, Electric.
A thermostat or apparatus, similar to a thermometer in some cases, for
closing an electric circuit when heated. It is used in connection with
automatic fire alarms to give warning of fire. For this use a
temperature of 52° C. (125° F.) is an approved one for setting one at,
to complete the circuit. It is also applied to regulation of
temperature, as in incubators.
(a) One kind of thermostat consists of a compound bar wound into a
spiral and fastened at one end, to which a terminal of a circuit is
connected. The bar may be made of two strips of brass and iron riveted
together, and wound into a spiral. When such a bar is submitted to
changes of temperature it bends in different directions, because brass
expands and contracts more under changes of temperature than does iron.
A contact point, to which the other terminal is connected, is arranged
to make contact with the spiral at any desired degree of temperature,
thus closing an electric circuit and ringing a bell, opening or closing
a damper, or doing anything else to notify an attendant or to directly
change the temperature.
If the brass forms the outside of the spiral, increase of temperature
makes the bending of the spiral bring the coils still closer. If the
brass forms the inside, increase of temperature makes the spiral tend to
become less close. As shown in the cut, the brass should lie along the
inside of the spiral.
Sometimes a straight compound bar is used, one of whose ends is fastened
and the other is free. As the temperature changes such a bar curves more
or less, its free end moving to and fro. Two contact screws are
provided, one on each side of its free end. If the temperature falls it
makes contact with one of these; if the temperature rises, it makes
contact with the other. Thus it may close one of two circuits, one for a
fall and the other for a rise in temperature.
It is well to introduce a third bar between the brass and iron ones,
made of some material of intermediate coefficient of expansion.
(b) Another kind of thermostat comprises a vessel of air or other gas,
which, expanding by heat, actuates a piston or other device and closes
an electric circuit. Synonym--Electro-pneumatic Thermostat.
(c) Another form utilizes the expansion of mercury. The mercury is made
part of an open electric circuit. As it expands it comes in contact with
the other terminal of the circuit, thus completing it, when the current
gives an alarm or does as is provided for in the apparatus employed.
Thermostats may be worked on either open or closed circuits; normally
the circuit may be open as described and may close on rise of
temperature, or it may be normally closed and open as the temperature
rises.
Fig. 335. ELECTRIC THERMOSTAT.