Thermometric Property

An instrument, a thermometer, is required to measure temperature objectively. The thermometer makes use of a physical property of a thermometric substance which changes continuously with temperature. The physical property is referred to as thermometric property.

Thermometric Properties Used In Various Thermometers

The following table shows some of the thermometric properties of matter that are used in the various thermometers:

Thermometric Property	Thermometer
Volume expansion of a gas	Gas thermometer
Volume expansion of a liquid	Laboratory or clinical thermometer
Volume expansion of a solid	Bi-metallic strip thermometer
Pressure change of a fixed mass of gas	Constant – volume gas thermometer
Changes in e.m.f.	Thermocouple
Changes in electrical resistance	Resistance thermometer or thermistor

Note: You can visit UY1: Measurement Of Temperature for a in-depth review of the different types of thermometers.

A good thermometric property of matter should vary:

• continuously with temperature
• uniquely over the the range of temperature to be measured
• its variation should be measurable.

Volume As Thermometric Property

Most solids or liquids or gases expand and contract their volume when the temperature around them changes.

Mercury is the only elemental metal that is liquid at ordinary temperatures. Mercury is silvery white and freezes into a soft solid like tin or lead at about -39 °C. Mercury does not wet glass or cling to it, and this property, coupled with its uniform volume expansion throughout its liquid range, makes it useful in liquid-in-glass thermometers.

Gases such as nitrogen or simply air are also suitable substances for use in volumetric gas thermometers. Typically, these gas thermometers work best at measuring very low temperatures.

Solid strips of different materials such as copper and aluminum can be made to stick together (bi-metallic strip). When the temperature of its surroundings changes, the different materials expand at different amounts, causing the bimetallic strip to bend, providing an indication to the change in temperature.

Advantages & Disadvantages of Mercury as thermometric substance

Advantages	Disadvantages
It is a good conductor of heat (High thermal conductivity)	Poisonous
Does not wet (cling to the sides) of the tube	Small thermal expansion
High boiling point ($357^{\circ}\text{C}$)	Expensive
Uniform expansion	High freezing point, $-39^{\circ}\text{C}$ (Cannot be used in places that are very cold)
Respond quickly to temperature changes
Visible meniscus

Advantages & Disadvantages of Alcohol as thermometric substance

Advantages	Disadvantages
Safe liquid	Wets the tube
Low freezing point ($-115^{\circ}\text{C}$)	Low boiling point ($78^{\circ}\text{C}$)
Large expansivity	Does not react quickly to temperature changes
Cheap	Needs to be dyed since it is colourless
	Non-uniform expansion

Advantages & Disadvantages Of Using Mercury Or Alcohol In Thermometers

Mercury	Alcohol
Silver	Colourless, usually dyed red
High thermal conductivity (Can respond very quickly to temperature changes)	Low thermal conductivity (Takes a longer time to reach its surrounding temperature)
Uniform expansion	Non-uniform expansion
Does not wet glass	Wets glass
Poisonous liquid	Safe liquid
Expensive	Cheap
Very dense liquid	Less dense liquid
Small thermal expansion (Scale markings on stem are very close to one another Hard to read)	Large thermal expansion (Scale marking on stem are far apart Easy to read)