The platinum resistance thermometer
This type of thermometer, devised
by Callendar in 1877, uses the change in the resistance of a platinum wire with temperature
to measure the change in temperature. The equation for such a change is:
where
q is the
temperature change and
a and
b
are constants,
b being much smaller than
a. We therefore ignore the term
b2 and assume that the resistance of the wire varies uniformly with temperature:
a is the temperature coefficient of resistance of the material.
For
platinum a = 3.8x10
-4 oC
-1.
A simple form of the platinum
resistance thermometer is shown in Figure 1. It consists of a platinum wire wound non-
inductively on a mica former and held in a glass tube by silica spacers. The resistance of the
wire is measured with a Wheatstone bridge network and to allow for the change in resistance
of the leads a set of dummy leads are included in the opposite arm of the bridge (see Figure
2).
This type of
thermometer has a large range, from -200
oC to +1100
oC and this can
be extended by the use of different wires. Bronze has a range starting at -260
oC
and using carbon temperatures as low as –270
oC can be measured.
The
advantages of the resistance thermometer are its convenient size, wide range and high
sensitivity (±0.000 05
oC). It can only be used for steady readings, however, and is
not direct-reading.
The accuracy obviously depends on how accurately the bridge
can be balanced.
