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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.
© Keith Gibbs 2007