Callister - An introduction - 8th edition

Chapter 19
Thermal Properties
(a)
(b)
One type of thermostat—a device that is used to regulate temperature—
utilizes the phenomenon of thermal expansion (the elongation of a material as it is
heated). The heart of this type of thermostat is a bimetallic strip—strips of two
metals, having different coefficients of thermal expansion, that have been bonded
along their lengths. A change in temperature will cause this strip to bend; upon
heating, the metal having the greater expansion coefficient will elongate more producing
the direction of bending shown in Figure (a). In these thermostats, the
bimetallic strip is as a coil or spiral [Figure (b)]; this configuration provides for a
relatively long bimetallic strip, more deflection for a given temperature change, (c)
and greater accuracy. The metal having the higher expansion coefficient is located on the underside of the strip such that,
upon heating, the coil tends to unwind. Attached to the end of the coil is a mercury switch—a small glass bulb that contains
several drops of mercury [Figure (b)]. This switch is mounted such that deflections of the coil end (when temperature is
changed) tips the bulb one way or the other; accordingly the blob of mercury will roll from one end of the bulb to the other.
When temperature reaches the set point of the thermostat, electrical contact is made as the mercury rolls to one end; this
switches on the heating or cooling unit (e.g., furnace or air conditioner).
The unit shuts off when a limit temperature is achieved and
as the bulb tilts in the other direction, the blob of mercury rolls to
the other end, and electrical contact is broken.
The photograph of Figure (d) shows the consequences of a
scorching heatwave that occurred in Melbourne, Australia: rail lines
buckled as a result of stresses from unanticipated thermal expansion.
[Figures (a) and (b) courtesy of Electrical-Forensics, Ray Franca,
PhD, PE. Photograph of Figure (c) from iStockphoto. Photograph of
Figure (d) from Calum Robertson/Newspix.]
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