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) |
its gud sha
Didn’t get the information needed do you mind tell me the thermometric properties of alcohol and mercury
Mi to
alcohol n mercury uses the thermometric property;
variation in volume expansion of liquid with temperature.
thanks I really need to understand physics
You are welcome l also got the information l wanted
Me too
Thanks
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How can define tampereture
Temperature is the degree of hotness and coldness of an object and it is a scalar quantity
Nice good job easy to comprehend im recommending this to others in my school
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is good to be study physics
But is too hard
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!!
Corrected the error. Thanks for pointing it out.
well summaries notes..
interesting notes but not all the informations
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good but not all the information i needed
Nice but not all the information I needed
really too much informative
Awesome information! 😀