Galileo is credited with inventing the first density-based thermometer,
the thermoscope. In an open container of alcohol, he placed a long glass tube
with a sphere at the upper end into this liquid after warming the sphere
with his hands. The air in the sphere would then cool and contract, drawing
alcohol into the tube. Then as the air temperature
changed, the air in the sphere would expand or contract causing the
colored alcohol to move up or down in the tube. Modern day Galileo
thermometers use sealed vials of various densities in a liquid. As
the temperature rises and the density of the liquid decreases, more of
the vials sink to the bottom. The temperature can be read by
marking each of the vials in turn.
Following the thermoscope, water-based, open-ended thermometers were
used. These were superceded by closed-glass liquid thermometers much
like those we use today. Mercury was chosen for precision thermometery
due to its uniform expansion.
In the mid 1700's, John Harrison invented the bi-metallic strip for
temperature compensation in clocks. Two metals with different
coefficients of expansion are bonded together. Heating them equally will
cause the strip to bend in one direction, cooling will cause them to
bend in the other direction. The deflection can be used to indicate
temperature. This device is widely used in thermostats and breaker
switches. Helical bi-metallic springs are used in dial thermometers.
The electrical resistance of metals is strongly temperature dependent
(the resistance increases with increasing temperature).
The most accurate thermometers use the resistance of platinum as the
measured indicator. The laboratory standard is known as the "bird-cage"
element. A helical platinum coil is suspended in a cage-like
configuration to minimize strain-induced resistance changes. A more
robust construction uses a film of platinum deposited on a ceramic
The resistance of a platinum thermometer is low (10-100 ohms) and the
temperature coefficient is also low (0.385 ohms/°C at 0°C) so the
resistance measuring equipment must be sensitive and accurate. The
resistance of the connecting wire must be minimized since it can have a
dramatic effect on the measurment. The usual method for measuring
resistance with with a Wheatstone Bridge where the platinum resistor
forms one arm of the bridge. A three wire bridge is used to control for
any temperature effects on the leads going to the thermometer.
Fig. 14.2.1 Three Wire Bridge
Resistance is converted to temperature using the following formula: