A BI-metallic strip works
on the principle that different metals have different coefficients
of expansion.
A BI-metallic strip consists of two strips of different
materials but of equal length. These two strips are riveted
together at different points along their lengths such that
they cannot slip sideways. Typically a welding process is
used for bonding, but rivets, bolts, adhesive and other
fasteners can also be used.
On heating the strip, the metal
having the higher coefficient of linear expansion expands
more than the other metal and therefore the strip bends.
The bending is such that the strip is concave on the side
of the metal of low coefficient of expansion and convex
on the side of the metal of large coefficient of expansion.
On cooling the strip, the reverse happens. Now, the metal
having a higher coefficient of expansion contracts more
and is therefore on the concave side while the metal of
low coefficient of expansion contracts less and is on the
convex side.
Example : A BI-metallic
strip is brass on one side and steel on the other. When
heated over a Bunsen burner the strip curves toward the
steel side. When cooled in liquid nitrogen, it curves the
other way.
Applications of BI-metallic strip:
Thermostat: A thermostat is a device used for maintaining
a steady temperature. It's basically a thermometer attached
to a switch, and it's job is to turn something on or off
when the thermometer reaches a certain temperature. Its
working is based on the behavior of a BI-metallic strip
with the change in temperature.
Often long BI-metallic strips
are coiled into spirals. By coiling a very long strip it
becomes much more sensitive to small temperature changes.
In a furnace thermostat, the same technique is used and
a mercury switch is attached to the coil. The switch turns
the furnace on and off.
Thermostats are used in many appliances such as electric
press, heater, fire alarm, flash unit, refrigerator, and
air conditioner etc.
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