# 2.2.2 Measurement of Temperature

### Measuring Temperature

• When a substance is heated, some of its physical properties can change
• These properties include things such as:
• The volume (and density) of the substance
• The electrical resistance of the substance
• If these properties change in a well-defined way, by measuring the property you can determine the temperature

• In order to build a thermometer based on one of these properties, you need to start by measuring the property at some well-defined fixed points
• A fixed point is a temperature at which some easily identifiable change occurs, such as the melting of ice (at 0 °c) or the boiling of pure water (at 100 °c)

Ice melts and water boils at well-defined temperatures (fixed points) which may be used to calibrate thermometers

• These fixed points allow you to know the temperature without having to measure it directly
• Usually two fixed points are used:
• The lower fixed point: The melting temperature of ice
• The upper fixed point: The boiling temperature of pure water
• Once a property (such as electrical resistance) has been measured at these two fixed points, the values of that property at other temperatures can be worked out

Measuring some property at two fixed points allows you to then work out the temperature for other values of that property

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### Sensitivity, Range & Linearity

• The above description has made a big assumption about the way in which the property changes with temperature: That it does so at a steady rate
• Properties that change in this way are said to be linear, and linearity is a very useful property for something to have as it makes it easy to work out the relationship between that thing and the temperature
• Other important properties when designing a device such as a thermometer include:
• Sensitivity – if the thing you are measuring changes significantly when the temperature changes, it makes it easier to detect small changes in temperature: Your thermometer is more sensitive
• Range – What are the lowest and highest temperatures the thermometer can measure? If a liquid-thermometer is too short, there may not be enough room for the liquid to expand into

### Liquid Thermometers

• A liquid-in-glass thermometer consists of a thin glass capillary tube containing a liquid that expands with temperature
• At one end of the tube is a glass bulb, containing a larger volume of the liquid which expands when heated, moving into the narrower tube
• 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

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### The Sensitivity of Liquid Thermometers

• The liquids used in liquid-in-glass thermometers are chosen for their thermal expansion properties:
• They expand linearly with temperature
• They expand significantly, giving them greater sensitivity
• They have low freezing points and high boiling points, allowing them to be used over a greater range of temperatures
• Sensitivity is another word for precision. Anything that makes it easier to detect small changes in temperature increases the precision, or sensitivity of a liquid thermometer
• Some of the factors which increase sensitivity are:
• Using a thermometer with a smaller glass bulb, as a smaller bulb contains less liquid and therefore, absorbs heat in a shorter time
• Using a narrow tube, as a small change in volume results in the liquid moving a larger distance along the tube
• Using a glass bulb with a thinner wall, as heat can be transferred to the bulb more easily and a small change in temperature can be more easily detected

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### Thermocouples

• A thermocouple consists of two different types of wire attached together
• When the joint between the two wires is heated, a potential difference (voltage) is created between the two wires
• The greater the temperature, the greater the potential difference

A thermocouple consists of two dissimilar wires connected together

• Thermocouples are not as sensitive as liquid-in-glass thermometers, but because the metals have high melting points, they can be used to measure very high temperatures
• Thermocouples are also very responsive to rapidly changing temperatures

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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