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has been that a complicated thermo-pile is not particularly durable. Even if this objection can be overcome, there remains the serious one that there are theoretical reasons, based upon thermodynamics (q.v.), for believing that the efficiency of the thermo-pile, as an instrument for converting heat energy into electrical energy, can never be high. In a particular case that was investigated by Lord Rayleigh, the maximum possible efficiency was found to be 6 per cent; and it is not likely that an efficiency materially greater than this will ever be actually attained with a thermo-pile that is large enough and durable enough to be of commercial value.

ALLAN D. RISTEEN, Director of Technical Research, The Travelers Insurance Company, Hartford, Conn.

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THERMOGRAPH (Greek, "heat-writing"),

form of self-registering thermometer (q.v.), by which an automatic record of variations of temperature is kept. Many different types of thermograph have been made, of which the following may be especially noted: (1) Photographic thermographs, in which the position of the mercury thread in an ordinary thermometer is photographed upon a moving sensitive film, either continuously or at short intervals of time; the moving sensitive film being actuated by clockwork, so that the time at which any given impression was made can be nicely determined. (2) Metallic-strip thermographs, in which a recording pen is actuated by a strip of metal composed of two substances of differing expansibility, riveted or soldered together. When a strip of this kind is heated, one of its sides expands more than the other, and the result is that the strip becomes curved by an amount which serves as a measure of the temperature to which the strip has been exposed. The pen which makes the record moves radially on a disc of paper which is caused to revolve at a steady rate by means of clockwork. (3) Electric-contact thermographs, in which a fine platinum wire is caused to descend, at intervals, into the open upper end of the capillary tube of a sensitive mercurial thermometer. When the wire touches the mercury column, it completes an electrical circuit, and by this means the position of the mercury thread in the thermometer is recorded. (4) The manometric thermograph, in which the pressure in a closed vessel filled with a gas is taken as the index of the temperature; the pressure being recorded automatically, and the temperature being afterward inferred from the recorded pressure, by means of a theoretical formula, or else by direct comparison of the instrument, at different temperatures, with a standard thermometer. (The pressure of an isolated mass of gas of this kind is known to be sensibly proportional to the absolute temperature of the gas, so long as the volume is kept constant).

THERMOMETER (Greek "heat-measurer"), an instrument for determining the temperatures of bodies. The general problem of thermometry is considered, in this encyclopedia, under the heading THERMOMETRY; and the present article will be devoted mainly to the consideration of the common mercury-in-glass form of the instrument, and to certain of its modifications. The mercury-in-glass thermom

eter depends for its action upon the fact that mercury expands about seven times as much as glass, for a given rise in temperature; so that when mercury is enclosed in a glass vessel, its apparent expansion is quite considerable. The mercury-in-glass thermometer consists essentially of four parts, these being (1) the mercury, (2) the spherical or cylindrical bulb of glass which contains it, (3) the fine tube which is attached to the bulb and which serves to make the expansion of the mercury evident, and (4) the graduated scale which is affixed to the capillary tube and from which the indications of the instrument are read. In the manufacture of thermometers which are intended for accurate work, the mercury is carefully purified by filtration through leather under pressure, and by subsequent distillation, and, in many cases, by chemical treatment also; and immediately before the mercury is placed in the thermometer it should be boiled so that it may be rendered free from moisture and from air. In the manufacture of the capillary stems of thermometers, some kind of glass which experience indicates to be adapted to this end is melted and the operator takes up a ball of it on the end of his blowpipe, blowing it out gradually and adding more glass to the mass from time to time. When the ball of molten glass has thus been brought to a convenient size, a second workman attaches his blowpipe to it also, and the two, still blowing, walk apart, so that the sphere of glass is drawn out into a very long and fine tube, which, when it has cooled, is cut into lengths and annealed. The calibre of each of these lengths is subsequently measured under the microscope, so that the instrument maker may know how large a bulb must be attached to each piece, in order that the degree-spaces on the finished thermometer may be of approximately the desired size. The bulb of the thermometer is usually made of a different kind of glass from the stem; and the process of making it consists simply in fusing to one end of the open stem a knob of special glass and then blowing it to size through the stem. In thermometers that are to be used for accurate scientific work, the bulbs should always be made of one of the three kinds of glass that are respectively known as "verre dur," Jena 16" and Jena 59. The first of these is a French glass, which has been demonstrated to be peculiarly adapted for use in thermometer bulbs, by the elaborate experiments made at the International Bureau of Weights and Measures, at Paris. The other two are made at Jena, Germany, and have been similarly proved to be adapted for use in accurate thermometers, by the experiments made at the Reichsanstalt, in Berlin. Since 1917 American glasses are manufactured that are well suited to the construction of accurate thermometers; but the problem to be solved is a difficult one and it will require much study and experiment. The stem and bulb of the thermometer being thus completed, the next step consists in cleaning them thoroughly on the inside. For this purpose they are washed out with hot nitric acid, with distilled water and with ether. They are then thoroughly dried, preferably by repeatedly exhausting them, while hot, with an air pump, and then filling them again with air that has been carefully dried. The next operation consists in filling the thermometer with pure mercury. In order

to do this the bulb is heated until the air that it contains is partially expelled and the open end of the stem is then dipped beneath the mercury. As the bulb cools, the air remaining within it contracts and mercury rises through the stem until the bulb has become partially filled; this operation being repeated until the bulb is full. The instrument is next heated to a temperature considerably higher than the highest temperature to which it is to be exposed in use, the mercury that it contains becoming thereby so much expanded that it fills the entire stem and runs over at the top; and while the stem is still filled in this manner it is sealed off at the end by means of a blowpipe. In the higher grades of thermometers, a tiny pear-shaped bulb is left at the top of the stem, partly as a precaution against the destruction of the thermometer in case it is accidentally exposed to too high a temperature in its subsequent service and partly as an aid in the calibration of the stem. When such a pear-shaped bulb is provided, the stem may be sealed off at the end while the internal space is exhausted by means of an air pump, instead of while it is filled with mercury; or the tube may be filled, above the mercury column, with dry nitrogen or some other inert gas. The glass part of the instrument having been completed, it remains to affix the scale to the stem. In high grade thermometers, the scale is engraved upon the stem directly; but in the cheaper forms it is usually engraved or stamped upon a piece of metal or of wood, to which the thermometer is finally secured. Let us consider the high grade instruments first and the cheaper ones afterward. Instruments of the former class are graduated by finding, experimentally, two definite points upon the stem, corresponding to two known temperatures; the two known temperatures which are selected for this purpose being the boiling point and freezing point of water. When these two points are found, the space between them is divided into a certain number of equal parts, which are called degrees. In determining the position of the boiling point upon a thermometer, the instrument is placed in steam that is rising from water that is boiling freely under a barometric pressure equal to that which would be produced by a column of pure, ice-cold mercury, 760 millimeters high, at sea-level in latitude 45°. When the mercury column in the thermometer ceases rising and becomes stationary, the point opposite which it stands is marked upon the stem and is called the "boiling point." If the barometric pressure under which the experiment is performed is not identically equal to the value assumed above, allowance must be made for that fact by the aid of the experiments of Regnault (or others) upon the variation of the boiling point of water per millimeter of change of barometric pressure. The boiling point having been marked upon the thermometer as here indicated, the instrument is then placed in a mixture of water and finely pulverized ice, as quickly as this can be safely done; and the point to which the mercury sinks is marked and called the "freezing point." The distance, on the stem, between the boiling and freezing points, is then marked off, by means of a dividing engine, into as many equal spaces as there are degrees between the freezing and boiling points of water and (save for the affixing of numbers to the degree-marks) the

thermometer is complete. It may be, of course, that the thermometer is to be divided into half degrees, or into tenths; but the operation is precisely the same, in this case, as it is when the division is to be made to degrees only.

We could evidently divide the space between the boiling point and the freezing point into as many equal "degrees" as we chose; for there is no reason, in the nature of things, why a "degree" could not have any one size, just as well as any other size. It is desirable, however, to have some uniform practice in this respect, and hence the manufacturers of thermometers invariably conform to one or the other of three standard systems. In France, and also for scientific work in nearly every country, it is customary to follow the plan introduced by Professor Celsius of Upsala, which consists in dividing the fundamental interval between the two fixed points into 100 equal parts, the freezing point being called "zero," or 0°, and the boiling point 100°. This method of graduation is known as the "Centigrade" (or "hundreddegree") system. For general purposes in the United States and in England, it is far commoner to graduate thermometers according to the system introduced by Fahrenheit of Dantzig, about 1714. In this system the interval between the freezing and boiling points is divided into 180 equal spaces, or "degrees"; but the freezing point is here called 32° and the boiling point 212° (32° 180° = 212°). There has been much discussion as to the reason that Fahrenheit had for dividing the fundamental interval into 180 equal parts; but there can be no doubt but that his zero point was intended to represent the greatest cold that was known in his day, this being obtained by mixing salt and snow. By adopting this lowest temperature as his zero, he probably sought to avoid the use of negative temperatures; but in these days when temperatures several hundred degrees lower than his zero can be produced, the significance of the 32 is lost, and we now adhere to it simply from custom. The third thermometric system that has been used to a considerable extent is that due to Réaumur. In this system the fundamental interval is divided into 80 equal degrees, and the freezing and boiling points are marked 0° and 80°, respectively. This method of graduation is extensively used in Germany upon thermometers intended for household purposes; but for most other purposes in that country it has given way to the Centigrade system.

After a thermometer that is to be used for precise measurement has been made and graduated, it is subjected to certain experimental investigations, for the purpose of ascertaining the errors to which it may be liable. One of the most important of these investigations relates to the "calibration error," which is due to such irregularities of calibre as the bore of the stem may possess. In order to determine the calibration errors, a thread of mercury of suitable length is detached temporarily from the column in the stem, by shaking the instrument. An expert in this kind of work can usually detach a thread of almost any length that he pleases, whether it be long or short. The instrument is then inclined so that the detached thread may be brought into various positions in the stem; and in certain of these positions its length is

observed with great care. The volume of the thread being constant, it is plain that its length will be greater where the calibre of the tube is small than it will be where the calibre is relatively large. The details of the operation of determining the calibration errors of a thermometer are very involved; but the general plan consists in observing the lengths of detached threads of mercury at different points of the stem and then computing from these observed lengths, the relative areas of cross-section of the stem-calibre at various points. It is then possible to calculate a table of calibration corrections, by the aid of which it will be easy to correct any given reading of the instrument, so as to find what reading would have been obtained if the item had been of absolutely uniform calibre throughout.

Prominent among the other sources of error, there are four that merit special attention. (1) In the measurement of a temperature, the bulb of the thermometer is supposed to be fully exposed to that temperature; but since the mercury in the stem must be seen in order to be read, it often happens that the stem of an instrument is necessarily exposed to conditions of temperature that are materially different from those to which the bulb is subjected. Hence there is often a "stem error" to a thermometer, due to the fact that the mercury thread in the stem is colder (or hotter) than that in the bulb, and, therefore, shorter (or longer) than it really ought to be. The magnitude of this stem error will obviously vary with the conditions under which the thermometer is used. It is always uncertain in amount and hence it is customary, in well-executed scientific work, to design the apparatus that is to be used (including the thermometer itself), with special reference to the desirability of keeping the stem error as small as possible. (2) When the barometric pressure upon the bulb of the thermometer varies, the bulb yields elastically to these variations and often to an extent quite sufficient to influence the reading of the instrument by an amount that cannot be neglected. The error due to this cause can be determined and eliminated by means of the "external pressure coefficient," which is obtained by subjecting the thermometer, at some fixed temperature, to a known change of external pressure and noting the alteration of the reading that this variation of pressure produces. (3) The pressure of the mercury upon the inner surface of the bulb may vary from several causes, one of which is the position of the thermometer itself. If the stem is in a vertical position, the bulb will be subjected to a pressure due to the height of the column of mercury in the stem; and when the thermometer is horizontal, this static pressure will be absent. In small thermometers the error due to this cause is unimportant; but in instruments of high precision, in which the stem may be several feet in length, it must receive due consideration. The constant which is used for correcting for this source of error and which is to be determined by experimenting with the thermometer in different positions but at the same constant temperature is called the "internal pressure coefficient." (4) It is found that the glass of which a thermometer is composed exhibits certain anomalies in its expansion and contraction, when its temperature is altered. These result in an apparent variation in the po

sition of the "zero point" of the thermometer, which is very troublesome when measurements of the highest precision are to be made. It is on account of this anomalous variation in the position of the zero point that the three kinds of glass mentioned in the earlier part of this article are recommended for the manufacture of the bulb; the variation of the zero having been studied in the case of these species of glass with great care. The phenomena as observed in the case of "verre dur" are thus described by Guillaume: "When a verre dur thermometer is quickly exposed to a temperature of 100° C., after having reposed for a considerable time at the ordinary temperature of the laboratory, its zero point falls with such rapidity that after an exposure of one minute at 100 C. the displacement is practically complete. If the thermometer is then placed in ice-water, its zero ascends, for the first few moments, at the rate of about 0.001° C. per minute; but this rate diminishes rapidly. When a thermometer is maintained at a constant temperature, its zero point rises little by little and the change can be traced plainly for several years. For thermometers of verre dur, the gradual rise at constant temperature amounts to about 0.001° C. per month when the thermometer is two years old; and at the end of four or five years the motion is found to have diminished to about 0.0002° C. per annum." The ideal way of measuring a temperature, with a thermometer made of one of the three glasses mentioned above, is as follows: The thermometer is exposed to the temperature that is to be measured, and its zero point falls to a certain (presumably unknown) position. After the instrument has been read, it is introduced, as quickly as is consistent with its safety, into a mixture of water and pulverized ice. The mercury sinks at once and soon attains a stable position, which, on account of the slowness of the change of zero with falling temperature, is taken to be the zero corresponding to the higher temperature to which the instrument has been previously exposed. In accordance with this plan, the temperature to be measured is found by subtracting the subsequent reading in ice-water from the reading obtained at the temperature to be determined. The method here outlined, for eliminating the effect of variations in the zero point of a thermometer, is known as the "method of movable zeros," and is now adopted at practically all of the centres of accurate thermometry except Kew, for temperatures between the freezing and boiling points. It is not yet possible, by any method of procedure, to determine temperatures more than a few degrees below the freezing point, or more than a hundred degrees (Centigrade) above the boiling point, by the aid of a mercury-in-glass thermometer, with a precision comparable with that which is attainable within the fundamental interval that lies between 0° C. and 100° C.

It is to be understood that in the foregoing discussion of the errors of the mercury-in-glass thermometer, we have been treating of the determination of temperatures to such a degree of precision that the final error is not to exceed (say) 0.005° C. No such elaborate care is required, if the only object of the measurement is

to determine the temperature to the nearest degree, or half-degree.

Passing now to the consideration of the ordinary thermometers that are used about the household and by amateur meteorological observers, it may be pointed out, first, that in the manufacture of a thermometer that is to be sold at retail for (say) 50 cents, it is not commercially possible to engrave a special scale for each instrument. In making cheap thermometers it is customary to stamp out the scales in large numbers and then to blow the bulb of each instrument to such a size that the scale will be as nearly as practicable adapted to the finished thermometer. This can be done, by an experienced glass-worker, with greater accuracy than might be supposed; but it is evident that no high degree of precision can be attained in this way. The scale and the rest of the thermometer being adapted to each other as nearly as is commercially practicable, the thermometer is adjusted with respect to the scale by exposing it to some known temperature (say 70° F.) in the vicinity of the temperatures at which it is most likely to be used and then securing it in such a position that the point on the stem to which the mercury rises comes opposite the proper mark on the scale. Such a theremometer will give readings that are not greatly in error at temperatures near the one at which it is standardized; but at other temperatures any two such thermometers will necessarily diverge by an amount which depends upon the judgment and skill of the workmen who blew the bulbs and who endeavored to give them capacities adapted to the sizes of the degrees upon their respective graduated scales.

For further information concerning the methods that are used in precise thermometry consult Guillaume, Thermometrie de Précision; and for the historical aspect of the subject consult H. Carrington Bolton, 'Evolution of the Thermometer.' Consult, also, Preston, 'Theory of Heat.'

Gas Thermometer.-A thermometer in which the temperature is measured by the change of volume, or pressure, of a mass of gas enclosed in a glass envelope. The gases that are most commonly employed for this purpose are air, hydrogen and nitrogen; and thermometers containing these several gases are respectively called "air thermometers," "hydrogen thermometers" and "nitrogen thermometers." See THERMOMETRY.

Alcohol Thermometer.-A thermometer in which the temperature is indicated by the expansion of alcohol (instead of mercury); coloring matter of some kind being dissolved in the alcohol, so that the column of fluid in the stem of the instrument may be distinctly visible. Alcohol has a larger coefficient of expansion than mercury, and hence, for the same sizes of bulb and stem, the degrees are longer upon a thermometer containing it. Alcohol can also be used at temperatures that are low enough to destroy an ordinary thermometer, by the freezing of the mercury. No great degree of precision can be attained with the alcohol thermometer, however, partly because the liquid wets the glass and thereby causes the instrument to read too low when the temperature is falling, and partly for other reasons. For the measurement

of temperatures approaching the freezing point of mercury (37.8° F. below zero) the International Bureau of Weights and Measures prefers a thermometer filled with toluene to one that is filled with alcohol; the toluene thermometer being apparently capable of yielding much more accurate results. Owing to the fact that alcohol boils at a much lower temperature than water, the alcohol thermometer can hardly be graduated by the method given for the mercury instrument, since exposure to a temperature of 212° F. would cause the alcohol to have a vapor pressure so high that the bulb would be likely to burst. These thermometers are, therefore, graduated, most commonly, by direct comparison with a standard mercury-inglass instrument. The expansion of alcohol by heat is not strictly proportional to that of mercury and hence if the scale of the mercury thermometer is taken as the standard, the degree marks upon the alcohol thermometer will not be spaced at uniform intervals. spaces are in fact smaller at low temperatures than at higher ones, as will be seen by examining any good alcohol thermometer that is adapted for observing a considerable range of temperature.

These

Maximum and Minimum Thermometers are thermometers which automatically record the highest or lowest temperatures to which they have been exposed during a given period. In the Rutherford maximum thermometer the capillary stem of the instrument is placed nearly horizontal and as the mercury rises it pushes before it a tiny index of iron or steel, placed within the tube; and the index, being left at the most extreme position attained by the mercury, indicates the highest temperature to which the instrument has been exposed. In the Rutherford minimum thermometer a similar index is used, but the thermometric column is here composed of alcohol and the index lies within the alcohol. When the temperature falls, the free end of the column of alcohol in the stem adheres to the index and drags it toward the bulb; but when the temperature rises again, the alcohol flows around the little index (which does not fill the capillary tube), and so leaves it in the position to which it had been drawn at the moment when the temperature was lowest. In both forms of thermometer the index is returned to a suitable position for making a new observation by the aid of a small magnet. In the Negretti and Zambra maximum thermometer the capillary tube is partially obstructed near the bulb so that although the mercury flows outward readily enough as the temperature rises, a fall of temperature at any moment causes the mercury thread in the stem to break at the obstruction, so that the maximum temperature to which the thermometer has been exposed can be read directly, in the usual manner. The broken thread can easily be returned to the partially empty bulb by jarring the instrument, or by whirling it sharply in a circle.

Clinical Thermometer.-A form of the Negretti and Zambra maximum thermometer, which is used by physicians for determining the temperature of the human body.

The graduation on these instruments is fine, so that the temperature can be read to the 10th of a degree or so; and the entire interval cov

ered by the graduation rarely extends below 95° F., or above 115° F., the normal temperature of the body being about 98° F. In using the instrument, the bulb is placed under the patient's tongue or in the arm-pit.

Radiation Thermometer.-A form of thermometer designed to indicate the intensity of solar or terrestial radiation. The solar radiation instrument consists of a thermometer with a blackened bulb, the stem being sealed into an exhausted sphere of glass, so that the blackened bulb comes in the centre of the sphere. When sunlight is allowed to fall upon this thermometer and also upon a similar one with a bulb that is silvered and polished, the black bulb absorbs most of the radiant heat, while the polished one reflects most of it. The difference in the readings of the two instruments is assumed to indicate the intensity of the radiant energy falling upon them.

Upsetting Thermometer.-A form of thermometer provided with a constriction in the stem similar to that used with the Negretti and Zambra maximum thermometer, and so designed that when the instrument is inverted the mercury thread breaks at the constriction and runs down into the stem. These instruments are graduated so as to read correctly when they are held upside down. By upsetting a thermometer of this kind by means of clockwork, the temperature that prevails at any particular hour can be recorded.

Deep-Sea Thermometer.-An instrument commonly of the upsetting type, for observing temperature at various depths in the sea. It is enclosed in a very strong case, and is reversed at the depth at which the temperature is desired. At moderate depths the reversal is effected by sending a weight down along the sounding wire; but at greater depths the upsetting mechanism is usually actuated by a small propeller which is arranged so as to begin its rotation when the thermometer starts on its return to the surface of the sea.

Registering Thermometer.-Any thermometer which automatically records its own readings.

Dew-Point Thermometer.-A thermometer adapted to the determination of the temperature at which dew will be deposited from the air. The most accurate form of the instrument is that devised by Regnault. This consists of a pair of thin receptacles of polished silver, shaped somewhat like ordinary chemical test-tubes. A thermometer is placed in each of these, and one of the tubes is then partially filled with ether, or some other volatile liquid. When a current of air is passed through the ether by means of an aspirator, the rapid evaporation cools the silver tube and its contents (including the thermometer); and the observation consists in noting the temperature of the ether, when the polished exterior of the silver tube containing it is first dimmed by the deposition of dew. The second tube of silver, which is not cooled, assists the eye in judging when the dew is first deposited upon the other one; and the thermometer that the uncooled tube contains is used merely to record the temperature of the air at the time of the experiment. Differential Thermometer.-An instrument for measuring or detecting differences of temperature, without reference to the absolute

values of the temperatures that are compared. Sir John Leslie's form, as improved by Rumford, consists of a horizontal tube, turned upward at the two ends, and there provided with a pair of equal bulbs of considerable size. The bulbs are filled with air, and a small quantity of colored liquid is placed in the horizontal tube which joins them; the liquid serving to separate the air masses that the bulbs contain, and also as an index for reading the instrument. So long as the temperatures of the two bulbs remain equal, the pressure of the air will be the same in each, and the liquid index will not move. If one of the bulbs is warmed slightly more than the other one, however, the air that it contains expands and forces the liquid index toward the cooler bulb; the amount of this displacement indicating the difference in the temperatures of the bulbs. This form of differential thermometer is not used to any great extent at the present time, the thermo-pile (see THERMO-ELECTRICITY) and the platinum resistance thermometer (see THERMOMETRY) having almost entirely displaced it.

Wet Bulb Thermometer.-A thermometer whose bulb is covered with thin wet muslin, and which is used for determining the amount of moisture in the air. In practice, the wet bulb thermometer is used in connection with a similar thermometer having a dry bulb, the two being whirled through the air together, or having a current of air directed upon them by a fan, or otherwise. The evaporation of the moisture about the wet bulb causes that instrument to become cooler than the other one; and the difference in the readings of the two thermometers, when taken in connection with the reading of the dry one, enables the observer to determine the degree of saturation of the air at the time the experiment is made. Tables for this purpose are published by the Weather Bureau.

Weight Thermometer.-A thermometer consisting of a bulb provided with a capillary outlet in the place of the usual stem. In using this instrument, the bulb is first weighed while empty, and again when filled with ice-cold mercury. It is next heated to the boiling point of water, and the mercury which escapes from it on account of the expansion is collected and weighed. These data enable the observer to calculate the fraction of the original weight of ice-cold mercury that is lost upon heating the bulb to the boiling point. To determine any other temperature, he fills the bulb, as before, with ice-cold mercury, and then exposes it to the temperature that is to be measured (this. temperature being assumed to be higher than the freezing point of water). Collecting the mercury that runs out of the bulb, and expressing its weight as a fraction of the weight of cold mercury that was present at the outset, he has only to compare the fraction so obtained with the fraction obtained in the first experiment, in order to be able to calculate, by a simple proportion, the temperature desired. The weight thermometer is not a convenient instrument to use, but it is simple in theory, and is free from certain of the errors to which ordinary thermometers are liable.

Metallic Thermometer.—An instrument in which temperature is determined by noting the change of form or of length that a metallic

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