1871: Resistance thermometer - The book of science

Resistance thermometer
How to calibrate
Resistance behavior
Commentary

Resistance thermometer

In 1821, Humphry Davy had observed
that conductance of metals varied according to temperature
but Davy hadn’t shown that this property
could be used to make a thermometer.
In 1871, Carl Wilhelm Siemens claimed that platinum
would be the best metal for a resistance thermometer,
having the most stable relation
between resistance and temperature over the greatest range.

How to calibrate

Adjust the sensor
to give an accurate reading
for a known state of a pure substance,
such as the triple point of water.
Adjust the sensor
to replicate the readings
of another sensor
that is known to be accurate.
Be sure to calibrate the sensor
over the range of temperatures
in which the sensor
is intended to be used.

Resistance behavior

When a delicate wire gets really hot,
it melts, making its resistance a moot question.
When it gets incredibly cold,
its resistance becomes independent of temperature.
Ideally, behavior depends on principles,
but in practice can randomly misbehave.

Resistance thermometers can be designed for temperatures between −328 and 932 °F. Outside of that range, resistance becomes non-linear or doesn’t depend on temperature. Most resistance thermometers are designed to work immersed in the environment or liquid that they measure; later, Samuel Pierpont Langley showed, with a similar design, the ability to determine a temperature at a distance.

Carl Wilhelm Siemens, without a criminal intent, also became known as Sir Charles William Siemens.