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 PropertyThermometer
Volume expansion of a gasGas thermometer
Volume expansion of a liquidLaboratory or clinical thermometer
Volume expansion of a solidBi-metallic strip thermometer
Pressure change of a fixed mass of gasConstant – volume gas thermometer
Changes in e.m.f.Thermocouple
Changes in electrical resistanceResistance 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.

Liquid Mercury

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.

Bimetallic strip
Bimetallic strip

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

AdvantagesDisadvantages
It is a good conductor of heat (High thermal conductivity)Poisonous
Does not wet (cling to the sides) of the tubeSmall thermal expansion
High boiling point ($357^{\circ}\text{C}$)Expensive
Uniform expansionHigh 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

AdvantagesDisadvantages
Safe liquidWets the tube
Low freezing point ($-115^{\circ}\text{C}$)Low boiling point ($78^{\circ}\text{C}$)
Large expansivityDoes not react quickly to temperature changes
CheapNeeds to be dyed since it is colourless
 Non-uniform expansion

Advantages & Disadvantages Of Using Mercury Or Alcohol In Thermometers

MercuryAlcohol
SilverColourless, 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 expansionNon-uniform expansion
Does not wet glassWets glass
Poisonous liquidSafe liquid
ExpensiveCheap
Very dense liquidLess 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)

Back To Thermal Physics (O Level)

Back To O Level Topic List


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20 thoughts on “Thermometric Property”

  1. Hey make a correction plz . In table showing alcoholic properties , u stated that it has uniform extension , but in table of comparison , u stated it has non uniform expension!!

    Reply

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