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Thermometers

Temperature

We can tell by our sense of touch when our body is warmer than another, but our senses are not always reliable. If you step out of a warm bath and place one foot on a cork mat and the other on a sheet of metal, you will be certain that the metal is much cooler than the cork, but of course you will be quite wrong. The cork and the metal must be at the same temperature, since they have been in the same room for a long time. What then is the explanation?

Later we shall see that a metal, like the above sheet, is a good conductor of heat and takes heat readily from your warm foot leaving it cold. On the other hand, cork is a very bad conductor for heat and hardly any heat passes from the foot placed on the cork mat, leaving the foot warm. Again if you come into a room, after walking on a cold night, the room seems quite warm, but to a person who has been sitting in the room for a time it may seem cold.

If we wish to find how hot a body is, or what we may call its degree of hotness, our sense of touch is not good enough. We do not usually speak of the degree of hotness of a body but rather of its temperature, and the instrument which measures and records the temperature is a thermometer.

Early Thermometers

It is not known for certain who made the first thermometer, but we generally agree that it was Galileo. The instrument which he invented in 1593 was not like our thermometers today. Other scientists made improvements and added new ideas. Galileo and Rey used water, a most unsuitable liquid, for their thermometer; Medici substituted wine for water, but only in 1724 Gabriel Fahrenheit, a native of Danzig, used mercury as a thermometric liquid and produced a really sound instrument. Celsius, who was professor of astronomy, finally made a further type in 1742.

Construction of a Liquid-in-Glass Thermometer

A glass tube, with a very fine bore, is attached to a glass bulb. This tube with a fine bore is sometimes called a capillary tube. Examine the broken end of an old thermometer with a hand lens: you will see that the bore is indeed as fine as a hair. Mercury fills the bulb and a portion of the capillary tube. The tube is sealed at the top after air has been removed. If the bulb is placed in warm surroundings, the liquid in the bulb expands and rises up the tube: it falls when the temperature of the surroundings falls. Thus to complete the thermometer, so that it is capable of measuring temperatures, we require a scale on which the position of the liquid may be recorded.

The Fixed Points

If we have two known temperatures marked on the thermometer, we can complete our scale by dividing, the space between the marks into an agreed number of divisions. The two special temperatures are called the fixed points. Nature has provided us with two temperatures which can be found very easily and which always remain the same.

a) The lower fixed point is the temperature at which pure water freezes, or pure ice melts.

b) The upper fixed point is the temperature at which pure water boils under standard atmosphere pressure (The pressure must be mentioned here because the boiling point of water varies as the pressure above the water varies. If the pressure is reduced, the boiling point of the water is reduced. It is a well-known fact that high upon Mt. Everest the atmospheric pressure is so low that water boils at only 75° C, and it is impossible to brew tea without the aid of some type of pressure cooker.).

These two points may be found quite accurately if we use very simple apparatus.

The lower fixed point is found by placing the bulb of the thermometer in melting ice. The ice must be melting since it might otherwise be at a temperature below freezing point. The ice must be pure. When the mercury in the tube has remained at a constant height for several minutes, a permanent mark is made on the stem.

The upper fixed point is found by placing the bulb in steam. Note: it is not placed in boiling water. Again the height of the mercury is marked when it is steady.

Now we have a thermometer with just two points marked on it and we proceed to make our scales.

Thermometer Scales

Numerical values are given to the two fixed points, values which will depend upon the scale which we decide to use. We shall consider only the two scales in common use, namely the scale given by Fahrenheit and the scale suggested by Celsius, which is called the Centrigrade scale.

- Lower fixed point Upper fixed point
Fahrenheit scale 32° F 212° F
Celsius scale 0°C 100°C

Each of the thermometers has two identical marks on the stem. In the case of the Celsius thermometer the lower mark is labelled 0° С and the upper 100° C. Divide the space between the two marks into 100 equal parts, calling each part 1 Celsius degree. This completes the Celsius scale between the fixed points. The scale may be extended by placing equal divisions below 0° С and above 100°C.

For the Fahrenheit thermometer, the two marks are labelled 32° F and 212° F. In this case there are 180 equal divisions between these two marks and each division will represent 1 Fahrenheit degree. This gives us the Fahrenheit scale.

Conversion of Scales

Again we see that two thermometers are identical except for the fact that they have different scales.

It will be necessary sometimes to convert a temperature on one scale to the corresponding temperature on the other scale. Later you will learn how to do this by using a formula (If the temperature on the Celsius scale is written as С and on the Fahrenheit scale as F, then the equation connecting the two scales is

F-32 = 9 .
С 5

)

Liquid Used in Thermometers

Mercury and alcohol are the two liquids most frequently used. Mercury has a high boiling point, 357° C, and a freezing point of 39° C. Thus it can be used for almost all temperatures met in ordinary laboratory experiments. Alcohol, however, boils at 78° С, which is far too low for simple experimental work, but its freezing point of 112° С is useful or registering temperatures in regions where winters may be very cold. Mercury may be seen easily, while alcohol must be coloured. Mercury does not wet glass as alcohol does - a distinct disadvantage of an alcohol thermometer when the temperature is falling.

Alcohol is usually used for cheap domestic thermometers and is also used in the maximum and minimum thermometers described below.

Special Thermometers

There are a number of different thermometers used for specific purposes.

Clinical Thermometer

This is a maximum thermometer constructed to read temperatures between 95° and 110°F. It is used by doctors to record the temperature of a patient's blood. The normal temperature of human blood is 98.4° F and changes of 2°F in temperature indicate that the patient is not in normal health. A clinical thermometer has a capillary tube, along which mercury passes and a small kink or constriction just above the bulb. Mercury expands up the tube when the bulb is placed under the patient's tongue. When the maximum temperature has been reached and the thermometer is removed from the mouth, the mercury column breaks at the constriction, leaving the top end stationary. Thus the patient's temperature may be read. The mercury is then brought back to the bulb, through the constriction, by shaking the thermometer briskly.


agreed number зд. согласованное число

as fine as hair тоненькое, как волосок

boiling point точка кипения

bore высверленное отверстие

bulb шарик

capable of measuring способный измерять

capillary tube капиллярная трубка

constriction сужение

freezing point точка замерзания

kink загиб, изгиб

leaving the foot warm зд. причем нога остается теплой

lens линза

mercury ртуть

- 112° С = minus 112 Centigrades

scale шкала

stem стержень

the top end верхний конец

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