in dimensions is not the same for all materials; it being
much greater in some

[Illustration: Simple Thermostat; Couple and Mounting Strip]

materials than in others, while in some there is practically no
change.

If two thin, narrow strips of different metals, that contract or
expand at different ratio due to a variation in temperature, be
rigidly fastened together at their ends, and the combination then
heated or cooled, the combined piece will have its shape changed.
One of the pieces will increase in length more than the other, due
to a rise in temperature, and this same piece will decrease in
length more than the other when subjected to a decrease in
temperature.

If one end of this combined piece be rigidly clamped to a support,
as shown in Fig. 1, and the combination then have its temperature
changed, the free end will move to the right or left of its
original position, depending upon which of the pieces changes in
length the more. If there is a rise in temperature and the
right-hand piece B increases in length faster than the left-hand
piece A, the free end of the combined piece win move to the left
of its original position. If, on the other hand, there is a
decrease in temperature, the right-hand piece will decrease in
length more than the left-hand piece, and the upper or free end
will move to the right of its original position.

Such a combination of two metals constitutes a simple thermostat.
If the movement of the free end of the combination be made to
actuate a needle moving over a properly calibrated scale we have a
simple form of thermometer. If two electrical contacts, CC, be
mounted on the right and left-hand sides of the upper end of the
combined piece, as shown in Fig. 1, we have a thermostat that may
be used in closing an electrical circuit when the temperature of
the room in which it is placed rises or falls a certain value.
These contacts should be so arranged that they can be moved toward
or away from the combined piece independently. By adjusting the
position of these contacts, the electrical circuit will be closed
when the temperature of the thermostat has reached an
experimentally predetermined value.

The following description is that of a thermostat, constructed by
the author of this article, which gave very satisfactory results.
First obtain a piece of steel, 6 in. long, 5/8 in. wide and 2/100
in. thick, and a piece of brass, 6 in. long, 5/8 in. wide and
3/100 in. thick. Clean on