Thermometers

The first thermometer was invented in 1592 by Galileo, who measured the expansion of air and found that its volume increases in proportion to its temperature if the pressure is kept constant (Note 1.M). His air thermometer tube was bent round, and so temperatures were described in 'degrees' around the circle.

In 1714, Fahrenheit used the expansion of mercury instead of air to obtain a less bulky method of temperature measurement. Mercury thermometers are widely used nowadays, though they have defects—markings wear off, bubbles can occur in the liquid, errors of observation arise unless the thermometer is read squarely (at right angles to the mercury), and there is a time-lag in taking up a steady value after a rapid temperature change. The lag is greatest in still air and, to allow for it, one should ventilate the thermometer and take repeated readings until three consecutive readings are the same. Finally, a broken thermometer releases mercury, whose fumes are dangerous to health.

Alcohol has several advantages over mercury. It expands about six times as much with heat, e.g. 1 per cent for a change of 10 K, and is much cheaper. But it too has disadvantages: the liquid may adhere to the walls of the thermometer's glass tube so that the main column reads too low; the column is easily disrupted during very hot weather or in transport, forming a bubble which falsely raises the apparent reading; the liquid may undergo a gradual chemical change and contract, thus giving a low reading. The tendency for alcohol thermometers to read too low contrasts with that of mercury thermometers, which tend to read too high.

Henry Cavendish invented a form of the mercury thermometer in 1757 which registers the highest temperature reached since the instrument was last reset. This is a maximum thermometer. As the temperature increases, mercury in the bulb expands through a constriction into the graduated thermometer stem (Figure 3.1). When the temperature falls, the

Separate Maximum And Minimum Thermometer
Figure 3 1 (a) A maximum thermometer, showing the constriction near the base and (b) a minimum thermometer, showing the meniscus and index. Both thermometers are shown in the usual horizontal position, to prevent the weight of the liquid affecting the reading.

column of mercury breaks at the constriction, where a small vacuum forms. Consequently, the reading remains at the highest value so far. The vacuum at the constriction is refilled only if the temperature later rises beyond the previous maximum. Resetting is done by shaking the mercury back into the bulb. If the thermometer is read and reset at 9 a.m. each day, the value recorded usually corresponds to conditions in the afternoon of the previous day.

A lowering of temperature in a minimum thermometer (Figure 3.1) leads to withdrawal of the alcohol surface towards the bulb. Surface tension then pulls on a lightweight metal index, which tends to rest against the walls of the almost horizontal thermometer. When temperatures rise, the alcohol expands past the index, which is left at the position of the lowest temperature since it was last reset. It is reset by means of a magnet.

Maximum and minimum mercury thermometers are combined in a design devised by James Six in 1782 (Figure 3.2). This is convenient and cheap, but less accurate than separate thermometers.

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