Imagens das páginas
PDF
ePub

-190]

Construction of the Scale.

187

liberated from the liquid, escapes by the lateral apertures. This steam is at the same temperature as the water from which it is liberated, and, when the mercury is stationary, a second mark is made upon the stem.

لسل ليلا

190. Construction of the scale.-Just as the foot-rule which is adopted as the unit of comparison for length is divided into a number of equal divisions called inches, for the purpose of having a smaller unit of comparison, so likewise the unit of comparison of temperatures, the range from zero to the boiling point, must be divided into a number of parts of equal capacity called degrees. There are three modes in which this is done. On the Continent, and more especially in France, this space is divided into 100 parts, and this division is called the Centigrade or Celsius scale; the latter being the name of the inventor. The Centigrade thermometer is almost exclusively adopted in foreign scientific works, and as its use is gradually extending in this country, it has been and will be adopted in this book.

The degrees are designated by a small cipher placed a little above on the right of the number which marks the temperature, and to indicate temperature below zero the minus sign is placed before them. Thus - 15° signifies 15 degrees below zero.

In accurate thermometers the scale is marked on the stem itself (fig. 165). It cannot be displaced, and its length remains fixed, as glass has very little inexpansibility. This is effected by covering the stem with a thin layer of wax, and then marking the divisions of the scale, as well as the corresponding numbers with a steel point. The thermometer is then exposed for about ten minutes to the vapours of a substance called hydrofluoric acid, which attacks the glass where the wax has been removed. The rest of the wax is then removed, and the stem is found to be permanently etched.

السياسسلسلس السيلين

Fig. 165.

Scales are also constructed on plates of ivory, wood, or metal, against which the stem is placed. Fig. 164 represents a mercury thermometer mounted on ivory; its scale extends from 20 degrees below zero to 110 degrees above.

Besides the Centigrade scale two others are frequently usedFahrenheit's scale and Réaumur's scale.

80

In Réaumur's scale the fixed points are the same as on the Centigrade scale, but the distance between them is divided into 80 degrees instead of into 100. That is to say, 80 degrees Réaumur are equal to 100 degrees Centigrade; one degree Réaumur is equal to 100 or of a degree Centigrade, and one degree Centigrade equals 100 or degree Réaumur. Consequently to convert any number of Réaumur degrees into Centigrade degrees (20 for example), it is merely necessary to multiply them by (which gives 25). Similarly, Centigrade degrees are converted into Réaumur's by multiplying them by

88

The thermometric scale invented by Fahrenheit in 1714 is still much used in England, and also in Holland and North America. The higher fixed point is like that of the other scales, the temperature of boiling water, but the null-point or zero is the temperature obtained by mixing equal weights of sal-ammoniac and snow, and the interval between the two points is divided into 212 degrees. The zero was selected because the temperature was the lowest then known, and was thought to represent absolute cold. When Fahrenheit's thermometer is placed in melting ice it stands at 32 degrees, and, therefore, 100 degrees on the Centigrade scale are equal to 180 degrees on the Fahrenheit scale, and thus I degree Centigrade is equal to degree Fahrenheit, and inversely 1 degree Fahrenheit is equal to 5 of a degree Centigrade.

If it be required to convert a certain number of Fahrenheit degrees (95 for example) into Centigrade degrees, the number 32 must first be subtracted, in order that the degrees may count from the same part of the scale. The remainder in the example is thus 63, and as I degree Fahrenheit is equal to 5 of a degree Centigrade, 63 degrees are equal to 63 x 5 or 35 degrees Centigrade.

If F be the given temperature in Fahrenheit's degrees and C the corresponding temperature in Centigrade degrees, the former may be converted into the latter by means of the formula

(F-32)=C,

and conversely, Centigrade degrees may be converted into Fahrenheit by means of the formula

C+32= F.

These formulæ are applicable to all temperatures of the two scales, provided the signs are taken into account.

Thus, to convert the

-192]

Alcohol Thermometer.

189

temperature of 5 degrees Fahrenheit into Centigrade degrees we have

[blocks in formation]

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. does not at first fill it

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 100 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 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

Fig. 167.

-194] Maximum and Minimum Thermometers.

191

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 10 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

[ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small]

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

« AnteriorContinuar »