# 17.1.3 Measurement of Temperature

### 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, 0oC and 100oC 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

#### 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

#### 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
• 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

#### Exam Tip

Remember to relate how the temperature is measured for different types of thermometer back to the scenario in the question. For example, make sure you say: the temperature increases as the volume of gas increases or the temperature increases as the e.m.f. between the two wires increases.

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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