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Thermometers

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Fig 1: Mercury and spirit thermometers with varying scales
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Summary

Devices used to measure temperature.

Operation

There are many different types of thermometer that can be used in school science. All measure temperature in one way or another. The main types are listed below.

Mercury-in-glass thermometer (see Fig 1)

Consists of a quantity of mercury in a sealed glass tube. Calibrated marks on the outside of the tube allow temperature to be read when the level of mercury reaches the line. A bulb one end contains most of the mercury. The contraction and expansion of this quantity is amplified within the capillary tube. The void is usually either a vacuum or filled with nitrogen. 'Spirit' thermometers have largely superseded mercury themometers because of their safer contents. These use a coloured liquid in place of the mercury

Galileo thermometer

Instantly recognisable by its wide glass tubing and floating (or sinking) weights. The tube is filled with a liquid which suspends varying masses. The liquid changes density with the temperature making the weights rise or fall to the point where their density matches that of the liquid. The lowest weight of the top set usually indicates the ambient temperature.

Infrared thermometer

These have the ability to measure temperatures of an object at a distance. Sometimes containing a laser which helps aim the device. The basic designs house a lens which focuses the infrared energy onto a detector. Once the ambient temperature is accounted for, the temperature is usually shown on an LCD screen. This type of thermometer is very useful to measure temperatures from the Leslie’s Cube as it is impractical to attach and then read from a standard thermometer.

Liquid crystal thermometer (see Figs 2 & 3)

These usually comprise a plastic strip housing liquid crystals that change colour when subjected to different temperatures. Disposable liquid crystal thermometers are available cheaply and can be used in conduction experiments where the thermometer is attached directly to the material being tested. Liquid crystal thermal radiation sensors are now available which help greatly in accurately measuring absorbed heat.

Safety

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