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189 temperature of 5 degrees Fahrenheit into Centigrade degrees we have
(5 – 32) 5 = –27 * 5 = -15C.
In like manner we have for converting Réaumur's into Fahrenheit's degrees the formula
R + 32 = F., and conversely, for changing Fahrenheit's into Réaumur's degrees, the formula
(F-32) = R. 191. Alcohol thermometer.—The alcohol thermometer differs from the mercurial thermometer in being filled with coloured alcohol. But as the expansion of liquids is less regular in proportion as they are near the boiling point, alcohol, which boils at 78° C., expands very irregularly. Hence, alcohol thermometers are usually graduated by placing them in baths at different temperatures together with a standard mercurial thermometer.
It is filled by gently heating the bulb, so as to repel a certain quantity of air, and then inverting it and plunging the open end of it into alcohol (fig. 166). The interior air contracts on cooling, and the atmospheric pressure raises the alcohol in the tube and in the bulb. It does not at first fill it completely, for some air
Fig. 166. . remains; but the alcohol is then boiled, and its vapours expel all the air; the tube is then again inverted and placed in alcohol, and now the instrument becomes quite filled. The further construction resembles that of a mercurial thermometer.
192. Limits to the employment of mercurial thermometers.Of all thermometers in which liquids are used, the one with mercury is the most useful, because this liquid expands most regularly, and is easily obtained pure, and because its expansion between – 36° and 100° is regular, that is, proportional to the degree of heat. It also has the advantage of having a very low specific heat. But for temperatures below – 36° C. the alcohol thermometer must be used, for mercury solidifies at - 40° C. to a mass like lead. Above joo degrees the coefficient of expansion increases and the indications of the mercurial thermometers are only approximate, the error amounting sometimes to several degrees. Mercurial thermometers also cannot be used for temperatures above 350°, for this is the boiling point of mercury. .
Observations by means of the thermometer. In taking the temperature of a room, the thermometer is usually suspended against the wall. This may, however, give rise to an error of several degrees; for if the wall communicates with the outside, and especially if it has a northern aspect, it will, generally speaking, be colder than the air in the room, and will communicate to the thermometer too low a temperature. On the other hand it may happen that the wall becomes too much heated by the sun's rays, or by chimney flues, and then the thermometer will be too high. The only way to obtain with accuracy the temperature of the air in a room is to
suspend the thermometer by a string in the centre, at a distance from any object which might raise or lower its temperature. The same remark applies to the determination of the temperature of the atmosphere; the thermometer must be suspended in the open air, in the shade, and not against a wall.
· 193. Leslie's differential thermometer.—Sir John Leslie constructed a thermometer for showing the difference of temperature of two neighbouring places, from which it has received the name differential thermometer. It consists of two glass bulbs containing air, and joined by a bent glass tube of small diameter fixed
on a frame (fig. 167). Before the apparatus Fig. 167.
is sealed, a coloured liquid is introduced
in sufficient quantity to fill the horizontal part of the tube, and about half the vertical legs. It is important to use a liquid which does not give off vapours at ordinary temperatures, and dilute sulphuric acid coloured with litmus is generally preferred. The apparatus being closed the air is passed
Maximum and Minimum Thermometers.
from one bulb into the other by heating them unequally until the level of the liquid is the same in both branches. A zero is marked at each end of the liquid column. To graduate the apparatus, one of the bulbs is raised to a temperature 10° higher than the other. The air of the first is expanded and causes the column of liquid, ba, to rise in the other leg. When the column is stationary the number 10 is marked on each side at the level of the liquid, the distance between zero and 10 being divided into no equal parts, both above and below zero, on each leg.
194. Rutherford's maximum and minimum thermometers. It is necessary, in meteorological observations, to know the highest temperature of the day, and the lowest temperature of the night. Ordinary thermometers could only give these indications by a continuous observation, which would be impracticable. Several instruments have accordingly been invented for this purpose, the simplest of which is Rutherford's. On a rectangular piece of plate glass (fig. 168) two thermometers are fixed, whose stems are bent hori
zontally. The one, A, is a mercurial, and the other, B, an alcohol thermometer. In A there is a small piece of iron wire, A, moving freely in the tube, which serves as an index. The thermometer being placed horizontally, when the temperature rises the mercury pushes the index before it. But as soon as the mercury contracts, the index remains in that part of the tube to which it has been moved, for there is no adhesion between the iron and the mercury. In this way the index registers the highest temperature which has been obtained; in the figure this is 31°. In the minimum thermometer there is a small hollow glass tube which serves as index. When it is at the end of the column of liquid, and the temperature falls, the column contracts and carries the index with it, in consequence of adhesion, until it has reached the greatest contraction. When the temperature rises, the alcohol expands, and passing between the sides of the tube and the index does not displace B. The position of the index gives therefore the lowest temperature which has been reached : in the figure this was 94 degrees below zero.
195. Pyrometers. The name pyrometers is given to instruments for measuring temperatures so high that mercurial thermometers could not be used. The older contrivances for this purpose, Wedgwood's, Daniell's (which in principle resembled the apparatus in fig. 156), Brongniart's, etc., are gone entirely out of use. None of them gives an exact measure of temperature.
RADIATION OF HEAT. 196. Radiant heat.-If we stand in front of a fire, or exposed to the sun's heat, we experience a sensation of warmth which is not due to the temperature of the air, for if a screen be interposed the sensation immediately disappears, which would not be the case if the surrounding air had a high temperature. Hence bodies can send out rays which excite heat, and which penetrate through the air without heating it, as rays of light through transparent bodies. Heat thus propagated is said to be radiated; and we shall use the terms ray of heat, or thermal, or calorific ray, in a similar sense to that in which we use the term ray of light, or luminous ray.
We shall find that the property of radiating heat is not confined to incandescent substances, such as a fire, or a lamp, or a red-hot ball, but that bodies of all temperatures radiate heat. Thus a bottle full of hot water and a bottle full of cold water both emit heat; the first emits more as compared with the second, the greater the difference of temperature between the two.
197. Laws of radiation.–The radiation of heat is governed by three laws.
I. Radiation takes place in all directions round a body. If a thermometer be placed in different positions round a heated body, it indicates everywhere a rise in temperature; at equal distances from the source of heat it indicates the same rise of temperature.
II. Heat is propagated in a right line. For, if a screen be placed
-198] Radiant Heat.
193 in the right line which joins the source of heat and the thermometer, so as to stop the rays, the latter is not affected.
But in passing obliquely from one medium into another, as from air into a glass, calorific like luminous rays become deviated, an effect known as refraction. The laws of this phenomenon are the same for heat as for light, and they will be more fully discussed under the latter subject.
III. Radiant heat is propagated in vacuo as well as in air. This is demonstrated by the following experiment.
In the bottom of a glass flask a thermometer is fixed in such a manner that its bulb occupies the centre of the flask (fig. 169). The neck of the flask is then carefully narrowed by means of the blowpipe,
Fig. 169. and then the apparatus having been suitably attached to an air-pump a vacuum is produced in the interior. This having been done, the tube is sealed at the narrow part. On immersing this apparatus in hot water, or on bringing near it some hot charcoal, the thermometer is at once seen to rise. This could only be due to radiation through the vacuum in the interior, for glass is so bad a conductor, that the heat could not travel with this rapidity through the sides of the flask, and the stem of the thermometer.
198. Causes which modify the intensity of radiant heat.The intensity of radiant heat transmitted to us by heated bodies depends on the temperature of the source of heat, and on its distance. The corresponding laws may be thus stated :
I. The intensity of radiant heat is proportional to the temperature of the source.
II. The intensity of radiant heat is inversely as the square of the distance.
The first law is demonstrated by placing a metallic box containing water at 10°, 20°, or 30°, successively, at equal distances from the bulb of a differential thermometer. The temperatures indicated by the latter are then found to be in the same ratio as those of the box : for instance, if the temperature of that corresponding to the box at 10° be 2°, those of the others will be 4o and 6° respectively.
The second law is demonstrated experimentally by placing the differential thermometer at a certain distance from the source of heat, a yard for instance, and then removing it to double the distance. In the latter case, the amount of heat received is not