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70 Chapter 4 • Heat


Mercury, on the other hand, has a high boiling point (360 °C) and is therefore used to measure relatively high temperatures such as those that occur when cooking.


Mercury is used in clinical and cooking thermometers while coloured alcohol, because of its cheapness, is used for swimming pool and room thermometers.


Ranges of thermometers vary according to their required application. The range of a clinical thermometer is usually between 35 °C and 42 °C, a swimming pool thermometer between 0 °C and 60 °C, those used formeasuring normal surrounding air temperature between –20 °C and 60 °C and cooking thermometers between 10 °C and 200 °C.


Table 4.1 shows the comparison between themercury and coloured alcohol thermometer. Compare


Mercury


Boiling point Freezing point Transparent


Colour


360 °C (advantage) –40 °C (disadvantage)


Coloured alcohol 80 °C (disadvantage) –130 °C (advantage)


No (advantage – easily visible) Yes (disadvantage – easily overcome by adding a colouring)


Silver


Sticks to the glass No (advantage) Cost


Transparent – adding different colours could be an advantage


Yes (disadvantage)


Uniformexpansion Yes (advantage) Table 4.1 Mercury versus alcohol


More expensive (disadvantage) Cheaper (advantage) No (disadvantage)


4.3.1 Construction and operation of a liquid in glass thermometer


Fig. 4.2 shows the construction of a liquid in a glass thermometer. The liquid can be either mercury or coloured alcohol, as their basic construction and principle of operation are the same.


Liquid Scale


Bulb Fig. 4.2 Termometer . 4.2 Termometer


Construction The glass rod has a uniformbore of small diameter. At the lower end the bore gives way to a relatively large bulb where the bulk of the mercury or coloured alcohol is stored.


Te walls of the bulb are thinner than the rest of the tube to allow for rapid heat transfer (conduction) to take place.


Glass


Bore of small diameter


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