Slike strani
PDF
ePub
[blocks in formation]

move.

ered by the graduation rarely extends below values of the temperatures that are compared. 95° F., or above 115° F., the normal tempera- Sir John Leslie's form, as improved by Rumture of the body being about 98° F. In using ford, consists of a horizontal tube, turned upthe instrument, the bulb is placed under the ward at the two ends, and there provided with patient's tongue or in the arm-pit.

a pair of equal bulbs of considerable size. The Radiation Thermometer.-A form of ther- bulbs are filled with air, and a small quantity mometer designed to indicate the intensity of of colored liquid is placed in the horizontal solar or terrestial radiation. The solar radia- tube which joins them; the liquid serving to tion instrument consists of a thermometer with separate the air masses that the bulbs contain, a blackened bulb, the stem being sealed into an and also as an index for reading the instrument. exhausted sphere of glass, so that the black- So long as the temperatures of the two bulbs ened bulb comes in the centre of the sphere. remain equal, the pressure of the air will be the When sunlight is allowed to fall upon this same in each, and the liquid index will not thermometer and also upon a similar one with

If one of the bulbs is warmed slightly a bulb that is silvered and polished, the black more than the other one, however, the air that bulb absorbs most of the radiant heat, while it contains expands and forces the liquid index the polished one reflects most of it. The dif- toward the cooler bulb; the amount of this ference in the readings of the two instruments displacement indicating the difference in the is assumed to indicate the intensity of the radi- temperatures of the bulbs. This form of difant energy falling upon them.

ferential thermometer is not used to any great Upsetting Thermometer.-A form of extent at the present time, the thermo-pile (see thermometer provided with a constriction in the

THERMO-ELECTRICITY) and the platinum resiststem similar to that used with the Negretti and ance thermometer (see THERMOMETRY) having Zambra maximum thermometer, and so designed

almost entirely displaced it. that when the instrument is inverted the mer

Wet Bulb Thermometer.-A thermometer cury thread breaks at the constriction and runs whose bulb is covered with thin wet muslin, down into the stem. These instruments are

and which is used for determining the amount graduated so as to read correctly when they

of moisture in the air. In practice, the wet are held upside down. By upsetting a ther- bulb thermometer is used in connection with a mometer of this kind by means of clockwork,

similar thermometer having a dry bulb, the two the temperature that prevails at any particular þeing whirled through the air together, or havhour can be recorded.

ing a current of air directed upon them by a Deep-Sea Thermometer.-An instrument fan, or otherwise. The evaporation of the . commonly of the upsetting type, for observing

moisture about the wet bulb causes that instrutemperature at various depths in the sea. It is

ment to become cooler than the other one; and enclosed in a very strong case, and is reversed

the difference in the readings of the two therat the depth at which the temperature is de

mometers, when taken in connection with the sired. At moderate depths the reversal is ef- reading of the dry one, enables the observer to fected by sending a weight down along the

determine the degree of saturation of the air sounding wire; but at greater depths the up

at the time the experiment is made. Tables setting mechanism is usually actuated by a small

for this purpose are published by the Weather propeller which is arranged so as to begin its

Bureau. rotation when the thermometer starts on its

Weight Thermometer.-A thermometer return to the surface of the sea.

consisting of a bulb provided with a capillary Registering Thermometer.-Any thermom

outlet in the place of the usual stem. In using eter which automatically records its own read- this instrument, the bulb is first weighed while ings.

empty, and again when filled with ice-cold merDew-Point Thermometer.—A thermometer cury. It is next heated to the boiling point adapted to the determination of the tempera

of water, and the mercury which escapes from it ture at which dew will be deposited from the

on account of the expansion is collected and air. The most accurate form of the instrument

weighed. These data enable the observer to calis that devised by Regnault. This consists of a

culate the fraction of the original weight of pair of thin receptacles of polished silver, shaped ice-cold mercury that is lost_upon heating the somewhat like ordinary chemical test-tubes. A

bulb to the boiling point. To determine any thermometer is placed in each of these, and one

other temperature, he fills the bulb, as before, of the tubes is then partially filled with ether,

with ice-cold mercury, and then exposes it to or some other volatile liquid. When a current

the temperature that is to be measured (this of air is passed through the ether by means of temperature being assumed to be higher than an aspirator, the rapid evaporation cools the

the freezing point of water). Collecting the silver tube and its contents (including the ther

mercury that runs out of the bulb, and expressmometer); and the observation consists in not- ing its weight as a fraction of the weight of ing the temperature of the ether, when the cold mercury that was present at the outset, he polished exterior of the silver tube containing has only to compare the fraction so obtained it is first dimmed by the deposition of dew. with the fraction obtained in the first experiThe second tube of silver, which is not cooled, ment, in order to be able to calculate, by a assists the eye in judging when the dew is first simple proportion, the temperature desired. The deposited upon the other one; and the thermome- weight thermometer is not a convenient instruter that the uncooled tube contains is used ment to use, but it is simple in theory, and is merely to record the temperature of the air at free from certain of the errors to which ordithe time of the experiment.

nary thermometers are liable. Differential Thermometer.-An instru- Metallic Thermometer.-An instrument in ment for measuring or detecting differences of which temperature is determined by noting the temperature, without reference to the absolute change of form or of length that a metallic

542

THERMOMETRIC ANALYSIS — THERMOMETRY

[ocr errors]

strip experiences when it is heated. In Bré- THERMO-ELECTRICITY), one of whose junctions guet's instrument, three thin strips composed is kept at a constant temperature, while the respectively of platinum, gold and silver are other is exposed to the temperature that is to rolled together into the form of a single rib- be measured. Of these four general methods, bon, the gold being in the centre. The the first two have been longest and most comribbon is then coiled into a spiral, with the monly employed; and the particular instrusilver on the concave side. When one end of ments that have been most extensively used for such a spiral is fixed, a rise of temperature putting them into practice are known respec. causes the spiral to partially unwind, owing to tively as the "mercury-in-glass thermometer the fact that the coefficient of expansion of and as the “gas thermometer.” The mercury silver is greater than that of gold, while the in-glass instrument is described under THER coefficient of gold is also greater than that of MOMETER, and the gas thermometer is describes platinum. The free end of the spiral is caused in the present article, below. to actuate a pointer, by which the temperature The gas thermometer was probably the firs is indicated.

form of thermometer to be constructed. Th Platinum Resistance Thermometer.-An mercury-in-glass instrument followed, and fo instrument for determining temperature, by many years was used almost exclusively for th noting the variation of the electrical resistance measurement of temperatures, doubtless on ac of a wire or strip of platinum. (See THER- count of its simplicity and the ease with whic! MOMETRY).

it can be used. But as the science of thermom ALLAN D. RISTEEN, etry developed, and increasing refinement i Director of Technical Research, The Travelers temperature determinations was demanded, Insurance Company, Hartford, Conn.

was found that the mercury-in-glass thermom THERMOMETRIC ANALYSIS. See eter is liable to serious errors on account of th CHEMICAL ANALYSIS.

anomalous expansions and contractions of th THERMOMETRY, the art of measuring glass envelope; errors which were of little temperatures. The "measurement of temper

no importance when a determination of ten ature is quite a different thing from the meas- perature to the nearest quarter of a degree urement of a time, or a length, or a mass, and

was considered sufficiently accurate, bu it consists merely in assigning to each temper

which were of paramount importance when ature that may come up for consideration a was proposed to determine a temperature definite place upon some sort of a numerical

the hundredth or thousandth of a degree. TH scale. The scale itself may be perfectly arbi

errors due to the cause in question can now ! trary, so that an interval of temperature upon

eliminated in large measure by making tempe one part of the scale cannot be said to be ature determinations by the "movable zerd "equal,” in any physical sense, to an interval on

method (see THERMOMETER); but physicis some other part, even though the two are ex- nevertheless prefer to follow the lead of Rey pressed by the same number of degrees. The nault, who, in his celebrated Fourth Memoi chief essentials of a practical thermemetric scale (1847), recommended the employment of th are (1) that it shall be perfectly definite, so gas thermometer as the standard for the e that when the same temperature is "measured tablishment of the temperature scale; and th on several different occasions, the same identi- gas thermometer is still the standard in cal result will be obtained each time, at least to work of high precision. The great advantaç a degree of approximation sufficient for the of the gas thermometer consists in the fa purposes for which the temperature is being

that the coefficients of expansion of gases a determined; and (2) that it shall be possible many times greater than that of mercury, ar for two or more different observers, provided

the effects of anomalous changes of size in tl with distinct instruments of measurement, to

glass bulb are of correspondingly less impor measure the same temperature, and obtain results that are identical, at least to the same The gas thermometer is made in two gei degree of approximation as noted above. So eral forms, according as it is desired to mea leng as these essential conditions are fulfilled, ure the temperature by the expansion of th we may make use, for the purpose of establish- gas at some constant pressure, or by the in ing a thermometric scale, of any measurable crease in the pressure of the gas at some coi property of matter, which varies in a determi- stant volume. The latter plan being the on nate way with temperature; the "temperature,” that is now by far the commoner in accura in any such case, being defined as proportional work, we shall describe it first, and at son to the atttribute that is measured, or to any length. continuous function of that attribute. We may, The constant-volume gas thermometer therefore, have as many different "scales of shown, in its essential features, in the accon temperature as we please, and any one of these panying illustration. It consists of a bulb, A, ( will be just as defensible, and just as "correct,” considerable size, which is connected, by mear as any other one, although no two of them of a capillary tube, with a mercury manomete will be in perfect agreement. In practice it is M. At a there is a mark upon the tube leadiri found that four particular kinds of thermo- to the gas bulb, and care is taken, wheneve metric scales are especially useful. These are an observation of any kind is made, to hav based, respectively, upon (1) the expansion of the level of the mercury in the short arm of th some substance that is subjected to an unvary- manometer stand exactly at a, in order that th ing pressure; (2) the increase in pressure of a volume of the thermometric gas may always ! gas which is kept rigorously constant in vol- rigorously the same. A movable reservoir o ume; (3) the variation of the electrical resist- mercury, V, is connected with the column 1 ance of a conductor; and (4) the electromo- for this purpose, by means of a flexible tube tive force of a thermo-electric couple (see so that by raising or lowering V the mercur

ance.

[blocks in formation]

in M may be brought to any desired level. Any the bulb A, then we have, from the definition gas that we please may be used in the bulb Å, of temperature, T=CP, where C is a constant but hydrogen, nitrogen and air are the ones for the particular thermometer under considmost commonly employed. In the filling of the eration. (It is to be observed that P is the bulb, the most elaborate precautions are taken, total pressure to which the gas in A is subnot only to have the gas that is used pure, but jected. It includes not only the pressure that also, and more particularly, to have it per- is read from the manometer M, but also that fectly dry. For this purpose the bulb is first barometric pressure that prevails at the same exhausted by the aid of an air-pump, and is time in the air of the laboratory; for this baroheated while in the exhausted condition, and metric pressure acts upon the top of the merallowed to stand for a time, so that any mois- cury column, and it is, therefore, to be added ture that may adhere to the walls of the bulb to the reading of the manometer M). To demay be driven off and removed. The bulb is duce the value of the constant C, we may subthen filled with gas that has been carefully ject the bulb A successively to the steam from dried by calcium chloride or other drying boiling water, and to a mixture of ice and agents, and is then exhausted again and heated; water, as described under THERMOMETER. The the operations of exhausting and refilling being total pressure upon the gas in the bulb being repeated several times, until there can be no noted in each case, let us suppose that it is Po doubt about the dryness and purity of the gas at the freezing point, and P100 at the boiling which is finally allowed to remain. Temper- point. Then the foregoing equation, when applied ature, according to this instrument, is defined to these two cases, takes the following forms, as being rigorously proportional to the pressure respectively: To=CP, T100= CP 100; To being that prevails in the bulb A, so long as the the temperature of the freezing point according

to the scale of this thermometer, and T100 being that of the boiling point. We may define either To or T100 however we please, and then find the corresponding value of C; but it is desir

able that the scale of the gas thermometer shall V

be as closely as possible like that of the ordinary mercury-in-glass instrument; and in order to fulfil this condition it is found to be best to subject the gas thermometer scale to the condition that the difference between T. and T 100, as determined by the gas thermometer, shall be numerically the same as the difference between the freezing and boiling points, on the ordinary mercury-in-glass scale. In other words, it is found to be best to have the average size of the degrees the same on the two

instruments. In scientific work the Centigrade M

scale is used in practically every instance; and if we adopt it here, we shall have the relation T 100— T.= 100°, if the condition just mentioned is to be fulfilled. From this and the preceding equations we easily find that C(P 100 P.) = 100°, or C=100/(P100 P.); so that when we know the values of P100 and Po by direct observation, we are prepared to determine C at once, and hence to calculate the

gastemperature, T, corresponding to any given pressure P, by means of the relation T=CP. It will be seen that the zero of the gas ther

mometer scale does not coincide with the freezА, ing point of water, but that it is very much

lower. The gas thermometer could not give T=0, for example, unless P=0; that is, not unless the temperature was so low as to cause the gaseous pressure to disappear altogether. The zero point from which the indications of the gas thermometer are counted, according to the formula given above, is called the "natural zero) of the instrument; and in order to be able to compare the gas scale with the scale of

the ordinary mercury-in-glass thermometer, it volume of the gas in the bulb remains constant. becomes necessary to know what the temperIt will be observed that there is here no as- ature of freezing water is, as read from the sumption that the thermometric gas obeys the gas scale. To determine this, we make use of laws of Boyle and Charles (see THERMODY- the relation To=CP.. Substituting in this the NAMICS); the relation which has just been as- value of C as already found, we find that sumed being the definition of the term “tem- Ti= 100P/ (P100 P.). Now the quantity perature, according to the constant-volume gas (P 100 P.)/P, is known as the coefficient of thermometer. If I be the temperature as thus expansion at constant volume) for the gas. defined, and P is the pressure prevailing within (The name is somewhat absurd, it is true, be

[graphic]
[blocks in formation]

pressure, at the freezing point of water, is equal to that due to a column of ice-cold mercury, one metre (1,000 mm.) high. The temperatures are understood to be «reduced,” as described above, so that the thermometer reads 0° at the freezing point and 100° at the boiling point. The ideal scale would of course be the absolute thermodynamic scale (see THERMODYNAMICS); but the corrections that are required in order to reduce gas thermometer readings to this scale are still too uncertain to be definitely adopted in precise thermometry. 2.- COMPARATIVE READINGS CONSTANT

VOLUME Gas THERMOMETERS AND THE MERCURY-VERRE DUR SCALE (“REDUCED) TEMPERATURES).

OF

[blocks in formation]

cause there is no expansion at all, if there is no change of volume; and it would be more accurate to designate this fraction as the coefficient of increase of pressure at constant volume). It appears, therefore, that the temperature of melting ice, on the scale of the constant-volume gas thermometer, is numerically equal to 100 times the reciprocal of the coefficient of expansion of the gas at constant volume. Having found To, we have only to subtract it from every reading of the gas thermometer, in order to reduce that reading to its corresponding value as reckoned from the freezing point of water. If we call the values of T To, as computed for any given gas thermometer, the (reduced readings of that thermometer, then we find that the reduced readings of the nitrogen, hydrogen, air and carbon dioxide constantvolume thermometers are all nearly identical, and that they are all closely comparable with the readings of the ordinary mercury-in-glass thermometer.

If two

constant-volume gas thermometers be filled with the same gas in different states of density, then the reduced readings of the two are very nearly equal, but yet not necessarily identical.

The coefficients of expansion at constant volume of certain of the more important thermometric gases are given in Table 1, as deduced from a careful analysis of the data given by Chappuis, Regnault and numerous other experimenters of high standing. The (initial pressure signifies the pressure on the gas in the thermometric bulb, when the bulb is surrounded by ice and water; this pressure being given as the most convenient way of fixing the density for which the coefficients were determined. Two coefficients are given for air at each initial pressure, because it appears to be 1.- COEFFICIENTS OF EXPANSION AT CONSTANT

VOLUME

[blocks in formation]

see

[blocks in formation]

In Table 2 comparative readings are given, of the mercury-in-glass («verre dur); THERMOMETER) scale, and the scales of the constant-volume hydrogen, nitrogen and carbon dioxide thermometers, in which the "initial pressures) are 1,000 millimetres of mercury. The significance of the table will be made plain by the following example: If all of these thermometers were exposed to a temperature at which the “reduced” reading of the hydrogen instrument was 30° C., then the nitrogen thermometer would read 30.011°, the carbon dioxide thermometer would read 30.054o and mercuryin-glass thermometer would read 30.102°. The readings given in the second column were obtained from experiments made upon the nitrogen thermometer; but Chappuis states that the reduced readings of the air thermometer and of the nitrogen thermometer are practically indistinguishable; and hence this column will serve for each of them.

In the constant-pressure gas thermometer, temperature is defined as proportional to the volume of a fixed mass of gas which is allowed to expand in such a manner that its pressure remains constant. Regnault experimented with thermometers of this class, and considered them to be distinctly inferior in accuracy, to those in which the volume is constant, and which we have already described. This judgment pronounced by Regnault has met with the approval of nearly every subsequent authority upon experimental physics, and hence the constantpressure gas thermometer has not been at all extensively used in practical work. Professor H. L. Callendar, in fact, is almost the only prominent advocate of the constant-pressure instrument at the present time. He claims that the constant-pressure gas thermometer is capable of yielding results even superior to those of the constant-volume thermometer; and

impossible to decide, from the observations thus far made, which one of these values is most likely to be correct, the available measures falling into two general groups, one of which favors one of the foregoing values, while the second favors the other one. In Table 1 the values of T, are also given, for convenience of reference.

The International Committee of Weights and Measures, in consideration of the differences that exist even between the reduced readings of constant-volume gas thermometers, adopted the following standard scale for the measurement of temperature, calling it their anormal thermometric scale. The scale adopted is the Centigrade scale of the constant-volume hydrogen thermometer, in which the hydrogen has a density such that its

[blocks in formation]
[ocr errors]

OF

GAS

273. 18°
272.43
268.48
272.44

he has devised a very ingenious form of the here computed, is the one that is commonly constant-pressure instrument, which certainly used, however; and Callendar and Harker and appears to overcome most of the objections that Chappuis have shown that the reduced platihave been urged against it in the past. (Con- num-temperature can be expressed in term of sult his paper entitled (On a Practical Ther- the reduced gas thermometer scale by means mometric Standard,' , in the Philosophical of a simple equation of the form: Magazine, for 1899, Vol. 48, page 519. Consult

T (Т also, Proceedings of the Royal Society, Vol.

pi=T + A. 50, 1892, page 247, and Preston, Theory of

100 100 Heat'). To facilitate computations connected with the constant-pressure gas thermometer, we

A being a constant whose value is to be deterpresent, in Table 3, the coefficients of expansion

mined experimentally. Callendar and Griffiths, of the principal thermometric gases at the con

for the purpose of determining A, recommend stant pressure of 1,000 millimeters of mercury

that the resistance of the platinum coil of the and also at 760 millimeters. These are obtained

thermometer be observed at the temperature of by a careful comparison of the best determina

boiling sulphur; the reduced temperature of tions that have yet been made.

this boiling point being, according to their ex

periments with the constant-pressure air ther3.- COEFFICIENTS OF EXPANSION

mometer, 444.53° C. (Eumorfopoulos states

GASES
AT CONSTANT PRESSURE.

that the boiling point of sulphur on this scale is
between 443.589 and 443.62° C. See (Proceed-

ings of the Royal Society,' 1908 A, 81, p. 339. Pressure = 1000 mm. Pressure = 760 mm.

Compare, also, Callendar and Moss, in the same

publication, 1909 A, 83, p. 106). The platinumCoefficient Coefficient

resistance thermometer gives great promise of of Το of

Το being a highly valuable instrument in the future. expansion expansion

Indeed, it is so already; but it does not yet appear to be capable of determining the abso

lute values of temperatures closer than to Hydrogen. 0.0036600 273. 22° 0.0036606

0.01° C. It may be used as a differential therNitrogen.

0.0036731 272.25 0.0036707 Carbon dioxide.. 0.0037422 267.22 0.0037247

mometer, however, so as to give results of a Air 0.0036734 272.23 0.0036706

far higher order of accuracy. For this pur

pose two similar coils or strips of platinum are The coefficient of expansion of carbon dioxid

used, these being placed in two of the arms of

a Wheatstone's bridge, so that the smallest deat a constant pressure of 760 millimeters of

parture from equality in their resistances can mercury must be considered as still somewhat uncertain, though the value given in the table

be observed. (See RESISTANCE, ELECTRICAL). appears to be the best now attainable. The Langley's bolometer is an instrument of this (natural zero of the constant-pressure ther

sort. It is used to explore the solar spectrum, mometer lies in about the same general region

and consists of two strips of platinum foil, as the natural zero of the constant-volume in

which are placed across the spectrum to be strument. The temperature of melting ice, as

examined, with their edges toward the source referred to the "natural zero of the scales of of the light. The two strips are placed in the the several constant-pressure gas thermometers,

two arms of a sensitive Wheatstone's bridge, is given in Table 3, in the columns headed

and so long as both the strips are exposed to «T..) No extensive and accurate comparisons

radiation of the same intensity, the balance of have yet been made between the constant-pres

the bridge is preserved. When one of the sure and constant-volume thermometers, either

strips coincides with a Fraunhofer line, howfor the same gas or for different ones.

ever, while the other is still exposed to the fuit In the platinum-resistance thermometer, tem

radiative power of the source of light, the perature is defined as proportional to the elec- balance is destroyed, and the existence of the trical resistance of a coil of pure, annealed

line is thereby demonstrated, even though the platinum wire. The thermometer) itself con

line be in the infra-red, where it is not visible to sists of a coil of the wire, wound upon a sheet or strip of mica, and placed in one of the arms

Thermo-electric couples have been used to a of a Wheatstone's bridge, so that its resistance

considerable extent for the measurement of may be accurately determined. It is usual to temperature, and Regnault experimented with denote a temperature as defined by the plati- them somewhat, but showed that they are disnum-resistance thermometer by the symbol "pp tinctly inferior in accuracy to the other known ("platinum temperature”). We have, therefore,

methods of determining temperature. At expt=CR, where R is the observed resistance of ceedingly low temperatures, however, they are the coil at the temperature denoted by pt and often of great value. Wroblewski, for example, Cis a constant whose value is to be determined. made use of thermo-couples quite extensively If Ro and pt, and Rim and pt100 are the respective for temperature measurements in his researches resistances and platinum-resistance tempera- on the critical points of the gases which are tures at the freezing and boiling points of liquefiable only at extremely low temperatures. water, then we have, precisely as in the case of The platinum resistance thermometer is more the constant-volume gas thermometer, pt= generally favored, however, for this purpose; 100 R/(R100 — Ro), as the platinum-resistance though it cannot be used for temperatures too temperature, as reckoned from the natural close to the absolute zero on account of the zero of the platinum-resistance thermometer. anomalous and sudden changes of resistance The (reduced platinum temperature, obtained that occur in that region. (See RESISTANCE, by subtracting pto from the temperature pt as

ELECTRICAL). At these extremely low temperaVOL. 26- 35

the eye.

« PrejšnjaNaprej »