Types of thermometers
For measurement of temperature number of thermometers are available using different
thermometric properties of the thermometric substances. Length, volume, pressure,
resistance, e.m.f. etc. are the commonly used thermometric properties for thermometers.
Different thermometers developed using these thermometric properties are given below.
1. Liquid in glass thermometer.
2. Constant volume gas thermometer.
3. Resistance thermometer.
4. Thermocouple.
(1) Liquid in glass thermometer
Liquid thermometers are those thermometers that employ liquids as the thermometric
substance and the change in volume of liquid with heat interaction is the characteristics
used for temperature measurement. Commonly used liquids in such thermometers are
Mercury and Alcohol. The figure shows the mercury in glass thermometer. In this the
change in volume of the mercury results in the rise or fall in the level of mercury column
in the glass tube. Out of the two liquids mercury is preferred over alcohol as it has low
specific heat and hence absorbs little heat from body. Mercury is comparatively a good
conductor of heat. Mercury can be seen in a fine capillary tube conveniently. Mercury
does not wet the wall of the tube. Mercury has a uniform coefficient of expansion over a
wide range of temperature and remains liquid over a large range as its freezing and boiling
points are –39ºC and 357°C respectively.
(2) Constant volume gas thermometer.
Thermometers using gaseous
thermometric substance are called gas
thermometers. Gas thermometers are
advantageous over the liquid thermometers
as the coefficient of expansion of gases is
more compared to liquids therefore these
are more sensitive. Also thermal capacity of
a gas is low compared to liquid so even a
small change can also be recorded
accurately. Gas thermometers are not
suitable for routine work as they are large,
cumbersome and can be used only in
certain fixed conditions. These are used
mainly for calibration and standardization
purpose.
The gas is contained in the bulb B which communicates with the mercury column M
through a capillary. The volume of the gas is kept constant by adjusting the height of the
mercury column M until the mercury level just touches the tip of a small pointer in the
space above M known as the dead space. The mercury column M is adjusted by raising
or lowering the reservoir. The difference in height between the two mercury columns is
measured when the bulb is surrounded by temperature t and when it is surrounded by
water at the triple point. The pressure of the gases is equal to atmospheric pressure ± h.
Temperature is given by t = 273.16(P/Pt).Where Pt = pressure of the gas at the triple point
of water. P= pressure of the gas at the temperature to be measured.
(3) Electrical resistance thermometer
Electrical resistance thermometer works on the
principle of change in resistance of the
thermometric substance (platinum) with
temperature. Thus resistance is the
thermometric property used in these
thermometers. It consists of a pure platinum wire
wound in a double spiral on a mica plate. Two
ends of the platinum wire are connected to the
copper leads (for low temperatures) or platinum
leads (for high temperatures). Principle of
Wheatstone bridge is employed in these
thermometers. The resistance is measured by
measuring a known constant current in the
thermometer and measuring the potential difference across the thermometer wire.
(4) Thermocouple
A thermocouple circuit is made
from joining two wires A & B
made of dissimilar metals. Due
to the seebeck effect a net
e.m.f. is generated in the circuit
which depends on the
difference in temperature
between the hot and cold
junctions. Hence the e.m.f. is a
thermometric property of the
circuit. This e.m.f. can be
measured by a micro voltmeter
to a high degree of accuracy.
The choice of metals depends
on the temperature range to be measured. The combination of metals in use are chromel
– alumel, copper – constantan, platinum – rhodium etc.
