14.1.3 Measurement of Temperature

• A thermometer is any device that is used to measure temperature
• Each type of thermometer uses a physical property of a material that varies with temperature - examples of such properties include:
  • The density of a liquid
  • The volume of a gas at constant pressure
  • Resistance of a metal
  • e.m.f. of a thermocouple

• In each case, the thermometer must be calibrated at two or more known temperatures (commonly the boiling and melting points of water, 0^oC and 100^oC respectively) and the scale divided into equal divisions

The Density of a Liquid

• A liquid-in-glass thermometer depends on the density change of a liquid (commonly mercury)
• It consists of a thin glass capillary tube containing a liquid that expands with temperature
• A scale along the side of the tube allows the temperature to be measured based on the length of liquid within the tube

As the bulb is heated, the liquid expands and moves along the capillary tube

Volume of a Gas at Constant Pressure

• The volume of an ideal gas is directly proportional to its temperature when at constant pressure (Charles’s law)

V ∝ T

• As the temperature of the gas increases, its volume increases and vice versa
• A gas thermometer must be calibrated - by knowing the temperature of the gas at a certain volume, a temperature scale can be determined depending on how quickly the gas expands with temperature

Resistance of a Metal

• Recall that electrical resistance changes with temperature e.g. the resistance of a filament lamp increases when current increases through it
  • For metals: resistance increases with temperature at a steady rate
  • For thermistors: resistance changes rapidly over a narrow range of temperatures

• As a thermistor gets hotter, its resistance decreases
• This means a thermometer based on a thermistor can be used to measure a range of temperatures
• The relationship between the resistance and temperature is non-linear
  • This means the graph of temperature against resistance will be a curved line and the thermistor will have to be calibrated

As the temperature of a thermistor increases, its resistance decreases

E.M.F. of a Thermocouple

• A thermocouple is an electrical device used as the sensor of a thermometer
• It consists of two wires of different, or dissimilar, metals attached to each other, producing a junction on one end
  • The opposite ends are connected to a voltmeter

• When this junction is heated, an e.m.f. is produced between the two wires which is measured on the voltmeter
• The greater the difference in temperature between the wires, the greater the e.m.f

A thermocouple consists of two dissimilar wires connected together

• However, a thermocouple requires calibration since the e.m.f. does not vary linearly with temperature
• The graph against e.m.f. and temperature is a positive, curved line

The e.m.f. and temperature are not directly proportional in a thermocouple