Figure 8-11.Expansion of a bar.
length (LO) at some temperature and is heated, it
will expand (Lf). The amount of expansion
is a function of the original length and the
temperature increase. The amount a material
changes in length with temperature is called the
linear coefficient of expansion.
The linear coefficient of expansion for a
material is a physical property of that material
and describes its behavior with respect to
If two materials with different linear coef-
ficients are bonded together, as the temperature
changes their rate of expansion will be different.
This will cause the entire assembly to bend in an
arc as shown in figure 8-12.
When the temperature is raised, an arc is
formed around the material with the smaller
expansion coefficient. Since this assembly is
formed by joining two dissimilar materials, it is
known as a bimetallic element.
A modification of this bimetallic strip serves
as the basis for one of the simplest and most
commonly encountered temperature-measuring
instruments, the bimetallic thermometer.
Figure 8-13 shows a bimetallic thermometer.
In it, a bimetallic strip is wound in the form of
a long helix. One end of the helix is held rigid.
As the temperature varies, the helix tries to wind
or unwind. This causes the free end to rotate. The
Figure 8-12.Effect of unequal expansion of a bimetallic
free end is connected to a pointer. The pointer
actually indicates angular rotation of the helix;
however, since the rotation is linear and a function
of temperature, the scale is marked in units of
Distant-reading dial thermometers are used
when the indicating portion of the instrument
must be placed at a distance from where the
temperature is being measured. The distant-
reading thermometer has a long capillary, some
Figure 8-13.Bimetallic thermometer.