Respiration Calorimeters for Studying the Respiratory Exchange and Energy Transformations of Man Part 2

As can be seen from these drawings the whole construction of the apparatus is more or less of the refrigerator type, _i. e._, there is little opportunity for radiation or conduction of heat. Such a construction could be multiplied a number of times, giving a greater number of insulating walls, and perhaps reducing radiation to the minimum, but for extreme accuracy in calorimetric investigations it is necessary to insure the absence of radiation, and hence we have retained the ingenious device of Rosa, by which an attempt is made arbitrarily to alter the temperature of the zinc wall so that it always follows any fluctuations in the temperature of the copper wall. To this end it is necessary to know _first_ that there is a temperature difference between zinc and copper and, _second_, to have some method for controlling the temperature of the zinc. Leaving for a moment the question of measuring the temperature differences between zinc and copper, we can consider here the methods for controlling the temperature of the zinc wall.

If it is found necessary to warm the zinc wall, a current of electricity is pa.s.sed through the resistance wire W, fig. 12. This wire is maintained approximately in the middle of the air-s.p.a.ce between the zinc wall and hair-felt by winding it around an ordinary porcelain insulator F, held in position by a threaded rod screwed into a bra.s.s disk soldered to the zinc wall. A nut on the end of the threaded rod holds the insulator in position. Much difficulty was had in securing a resistance wire that would at the same time furnish reasonably high resistance and would not crystallize or become brittle and would not rust. At present the best results have been obtained by using enameled manganin wire. The wire used is No. 28 American wire-gage and has resistance of approximately 1.54 ohms per foot. The total amount of wire used in any one circuit is equal to a resistance of approximately 92 ohms. This method of warming the air-s.p.a.ce leaves very little to be desired. It can be instantaneously applied and can be regulated with the greatest ease and with the greatest degree of refinement.

If, on the other hand, it becomes necessary to cool the air-s.p.a.ce next to the zinc and in turn cool the zinc, we must resort to the use of cold water, which is allowed to flow through the pipe C suspended in the air-s.p.a.ce between the zinc and hair-felt at approximately the same distance as is the heating wire. The support of these pipes is accomplished by placing them in bra.s.s hangers G, soldered to the zinc and provided with an opening in which the pipe rests.

In the early experimenting, it was found impracticable to use piping of very small size, as otherwise stoppage as a result of sediment could easily occur. The pipe found best adapted to the purpose was the so-called standard one-eighth inch bra.s.s pipe with an actual internal diameter of 7 millimeters. The opening of a valve allowed cold water to flow through this pipe and the considerable ma.s.s of water pa.s.sing through produced a very noticeable cooling effect. In the attempt to minimize the cooling effect of the ma.s.s of water remaining in the pipe, provision was made to allow water to drain out of this pipe a few moments after the valve was closed by a system of check-valves. In building the new apparatus, use was made of the compressed-air service in the laboratory to remove the large ma.s.s of cold water in the pipe. As soon as the water-valve was closed and the air-c.o.c.k opened, the compressed air blew all of the water out of the tube.

[Ill.u.s.tration: FIG. 13.--Detail of drop-eight feed-valve and arrangement of outside cooling circuit. The water enters at A, and the flow is regulated by the needle-valve at left-hand side. Rate of flow can be seen at end of exit tube just above the union. The water flows out at C and compressed air is admitted at B, regulated by the pet-c.o.c.k.]

The best results have been obtained, however, with an entirely new principle, namely, a few drops of water are continually allowed to pa.s.s into the pipe, together with a steady stream of compressed air. This cold water is forcibly blown through the pipe, thus cooling to an amount regulated by the amount of water admitted. Furthermore, the relatively dry air evaporates some of the water, thereby producing a somewhat greater cooling effect. By adjusting the flow of water through the pipe a continuous cooling effect of mild degree may be obtained. While formerly the air in the s.p.a.ce next the zinc wall was either cooled or heated alternately by opening the water-valve or by pa.s.sing a current through the heating coil, at present it is found much more advantageous to allow a slow flow of air and water through the pipes continuously, thus having the air-s.p.a.ce normally somewhat cooler than is desired. The effect of this cooling, therefore, is then counterbalanced by pa.s.sing an electric current of varying strength through the heating wire. By this manipulation it is unnecessary that the observer manipulate more than one instrument, namely, the rheostat, while formerly he had to manipulate valves, compressed-air c.o.c.ks, and rheostat. The arrangement for providing for the amount of compressed air and water is shown in fig. 13, in which it is seen that a small drop-sight feed-water valve is attached to the pipe C leading into the dead air-s.p.a.ce surrounding the calorimeter chamber. Compressed air enters at B and the amount entering can be regulated by the pet-c.o.c.k. The amount of water admitted is readily observed by the sight feed-valve. When once adjusted this form of apparatus produces a relatively constant cooling effect and facilitates greatly the manipulation of the calorimetric apparatus as a whole.

THE THERMO-ELECTRIC ELEMENTS.

In order to detect differences in temperature between the copper and zinc walls, some system for measuring temperature differences between these walls is essential. For this purpose we have found nothing that is as practical as the system of iron-German-silver thermo-electric elements originally introduced in this type of calorimeter by E. B.

Rosa, of the National Bureau of Standards, formerly professor of physics at Wesleyan University. In these calorimeters the same principle, therefore, has been applied, and it is necessary here only to give the details of such changes in the construction of the elements, their mounting, and their insulation as have been made as a result of experience in constructing these calorimeters. An element consisting of four pairs of junctions is shown in place as T-J in fig. 25.

One ever-present difficulty with the older form of element was the tendency for the German-silver wires to slip out of the slots in which they had been vigorously crowded in the hard maple spool. In thus slipping out of the slots they came in contact with the metal thimble in the zinc wall and thus produced a ground. In constructing the new elements four pairs of iron-German-silver thermal junctions were made on essentially the same plan as that previously described,[6] the only modification being made in the spool. While the ends of the junctions nearest the copper are exposed to the air so as to take up most rapidly the temperature of the copper, it is somewhat difficult to expose the ends of the junctions nearest the zinc and at the same time avoid short-circuiting. The best procedure is to extend the rock maple spool which pa.s.ses clear through the ferule in the zinc wall and cut a wide slot in the spool so as to expose the junctions to the air nearest the ferule. By so doing the danger to the unprotected ends of the junctions is much less. The two lead-wires of German silver can be carried through the end of the spool and thus allow the insulation to be made much more satisfactorily. In these calorimeters free use of these thermal junctions has been made. In the chair calorimeter there are on the top 16 elements consisting of four junctions each, on the rear 18, on the front 8, and on the bottom 13. The distribution of the elements is made with due reference to the direction in which the heat is most directly radiated and conducted from the surface of the body.

While the original iron-German-silver junctions have been retained in two of these calorimeters for the practical reason that a large number of these elements had been constructed beforehand, we believe it will be more advantageous to use the copper-constantin couple, which has a thermo-electric force of 40 microvolts per degree as against the 25 of the iron-German-silver couple. It is planned to install the copper-constantin junctions in the calorimeters now building.

INTERIOR OF THE CALORIMETER.

Since the experiments to be made with this chamber will rarely exceed 6 to 8 hours, there is no provision made for installing a cot bed or other conveniences which would be necessary for experiments of long duration.

Aside from the arm-chair with the foot-rest suspended from the balance, there is practically no furniture inside of the chamber, and a shelf or two, usually attached to the chair, to support bottles for urine and drinking-water bottles, completes the furniture equipment. The construction of the calorimeter is such as to minimize the volume of air surrounding the subject and yet secure sufficient freedom of movement to have him comfortable. A general impression of the arrangement of the pipes, chair, telephone, etc., inside the chamber can be obtained from figs. 7 and 9. The heat-absorber system is attached to rings soldered to the ceiling at different points. The incoming air-pipe is carried to the top of the central dome, while the air is drawn from the calorimeter at a point at the lower front near the position of the feet of the subject.

From this point it is carried through a pipe along the floor and up the rear wall of the calorimeter to the exit.

With the perfect heat insulation obtaining, the heat production of the man would soon raise the temperature to an uncomfortable degree were there no provisions for withdrawing it. It is therefore necessary to cool the chamber and, as has been pointed out, the cooling is accomplished by pa.s.sing a current of cold water through a heat-absorbing apparatus permanently installed in the interior of the chamber. The heat-absorber consists of a continuous copper pipe of 6 millimeters internal diameter and 10 millimeters external diameter. Along this pipe there are soldered a large number of copper disks 5 centimeters in diameter at a distance of 5 millimeters from each other. This increases enormously the area for the absorption of heat. In order to allow the absorber system to be removed, added to, or repaired at any time, it is necessary to insert couplings at several points. This is usually done at corners where the attachment of disks is not practicable. The total length of heat-absorbers is 5.6 meters and a rough calculation shows that the total area of metal for the absorption of heat is 4.7 square meters. The total volume of water in the absorbers is 254 cubic centimeters.

It has been found advantageous to place a simple apparatus to mix the water in the water-cooling circuit at a point just before the water leaves the chamber. This water-mixer consists of a 15-centimeter length of standard 1-inch pipe with a cap at each end. Through each of these caps there is a piece of one-eighth-inch pipe which extends nearly the whole length of the mixer. The water thus pa.s.sing into one end returns inside the 1-inch pipe and leaves from the other. This simple device insures a thorough mixing.

The air-pipes are of thin bra.s.s, 1-inch internal diameter. One of them conducts the air from the ingoing air-pipe up into the top of the central dome or hood immediately above the head of the subject. The air thus enters the chamber through a pipe running longitudinally along the top of the dome. On the upper side of this pipe a number of holes have been drilled so as to have the air-current directed upwards rather than down against the head of the subject. With this arrangement no difficulties are experienced with uncomfortable drafts and although the air enters the chamber through this pipe absolutely dry, there is no uncomfortable sensation of extreme dryness in the air taken in at the nostrils, nor is the absorption of water from the skin of the face, head, or neck great enough to produce an uncomfortable feeling of cold.

The other air-pipe, as suggested, receives the air from the chamber at the lower front and pa.s.ses around the rear to the point where the outside air-pipe leaves the chamber.

The chamber is illuminated by a small gla.s.s door in the food aperture.

This is a so-called "port" used on vessels. Sufficient light pa.s.ses through this gla.s.s to enable the subject to see inside the calorimeter without difficulty and most of the subjects can read with comfort. If an electric light is placed outside of the window, the illumination is very satisfactory and repeated tests have shown that no measurable amount of heat pa.s.ses through the window by placing a 32 c. p. electric lamp 0.5 meter from the food aperture outside. More recently we have arranged to produce directly inside the chamber illumination by means of a small tungsten electric lamp connected to the storage battery outside of the chamber. This lamp is provided with a powerful mirror and a gla.s.s shade, so that the light is very bright throughout the chamber and is satisfactory for reading. It is necessary, however, to make a correction for the heat developed, amounting usually to not far from 3 calories per hour.

By means of a hand microphone and receiver, the subject can communicate with the observers outside at will. A push-b.u.t.ton and an electric bell make it possible for him to call the observers whenever desired.

HEAT-ABSORBING CIRCUIT.

To bring away the heat produced by the subject, it is highly desirable that a constant flow of water of even temperature be secured. Direct connection with the city supply is not practicable, owing to the variations in pressure, and hence in constructing the laboratory building provision was made to install a large tank on the top floor, fed with a supply controlled by a ball-and-c.o.c.k valve. By this arrangement the level in the tank is maintained constant and the pressure is therefore regular. As the level of the water in the tank is approximately 9 meters above the opening in the calorimeter, there is ample pressure for all purposes.

[Ill.u.s.tration: FIG. 14.--Schematic diagram of water circuit for heat-absorbers of calorimeter. A, constant-level tank from which water descends to main pipe supplying heat-absorbers; _a_, valve for controlling supply from tank A; B, section of piping pa.s.sing into cold brine; _b_, valve controlling water direct from large tank A; _c_, valve controlling amount of water from cooling section B; C, thermometer at mixer; D, electric heater for ingoing water; E, thermometer for ingoing water; _d d d_, heat-absorbers inside calorimeter; F, thermometer indicating temperature of outcoming water; G, can for collecting water from calorimeter; _f_, valve for emptying G.]

The water descends from this tank in a large 2-inch pipe to the ceiling of the calorimeter laboratory, where it is subdivided into three 1-inch pipes, so as to provide for a water-supply for three calorimeters used simultaneously, if necessary, and eliminate the influence of a variation in the rate of flow in one calorimeter upon the rate of flow in another.

These pipes are brought down the inner wall of the room adjacent to the refrigeration room and part of the water circuit is pa.s.sed through a bra.s.s coil immersed in a cooling-tank in the refrigeration room. By means of a by-pa.s.s, water of any degree of temperature from 2 C. to 20 C. may be obtained. The water is then conducted through a pipe beneath the floor to the calorimeter chamber, pa.s.sed through the absorbers, and is finally measured in the water-meter.

A diagrammatic sketch showing the course of the water-current is given (fig. 14), in which A is the tank on the top floor controlled by the ball c.o.c.k and valve, and _a_ is the main valve which controls this supply to the cooler B, and by adjusting the valve _b_ and valve _c_ any desired mixture of water can be obtained. A thermometer C gives a rough idea of the temperature of the water, so as to aid in securing the proper mixture. The water then pa.s.ses under the floor of the calorimeter laboratory and ascends to the apparatus D, which is used for heating it to the desired temperature before entering the calorimeter. The temperature of the water as it enters the calorimeter is measured on an accurately calibrated thermometer E, and it then pa.s.ses through the absorber system _d d d_ and leaves the calorimeter, pa.s.sing the thermometer F, upon which the final temperature is read. It then pa.s.ses through a pipe and falls into a large can G, placed upon scales. When this can is filled the water is deflected for a few minutes to another can and by opening valve _f_ the water is conducted to the drain after having been weighed.

_Brine-tank._--The cooling system for the water-supply consists of a tank in which there is immersed an iron coil connected by two valves to the supply and return of the brine mains from the central power-house.

These valves are situated just ahead of the valves controlling the cooling device in the refrigeration room and permit the pa.s.sage of brine through the coil without filling the large coils for the cooling of the air in the calorimeter laboratory. As the brine pa.s.ses through this coil, which is not shown in the figure, it cools the water in which it is immersed and the water in turn cools the coil through which the water-supply to the calorimeter pa.s.ses. The bra.s.s coil only is shown in the figure. The system is very efficient and we have no difficulty in cooling the water as low as 2 C. As a matter of fact our chief difficulty is in regulating the supply of brine so as not to freeze the water-supply.

_Water-mixer._--If the valve _b_ is opened, water flows through this short length of pipe much more rapidly than through the long coil, owing to the greater resistance of the cooling coil. In conducting these experiments the valve c is opened wide and by varying the amount to which the valve _b_ is opened, the water is evenly and readily mixed.

The thermometer C is in practice immersed in the water-mixer constructed somewhat after the principle of the mixer inside the chamber described on page 21. All the piping, including that under the floor, and the reheater D, are covered with hair-felt and well insulated.

_Rate-valves._--It has been found extremely difficult to secure any form of valve which, even with a constant pressure of water, will give a constant rate of flow. In this type of calorimeter it is highly desirable that the rate of flow be as nearly constant as possible hour after hour, as this constant rate of flow aids materially in maintaining the calorimeter at an even temperature. Obviously, fluctuations in the rate of flow will produce fluctuations in the temperature of the ingoing water and in the amount of heat brought away. This disturbs greatly the temperature equilibrium, which is ordinarily maintained fairly constant.

Just before the water enters the reheater D it is caused to pa.s.s through a rate-valve, which at present consists of an ordinary plug-c.o.c.k. At present we are experimenting with other types of valves to secure even greater constancy, if possible.

_Electric reheater._--In order to control absolutely the temperature of the water entering at E, it is planned to cool the water leaving the water-mixer at C somewhat below the desired temperature, so that it is necessary to reheat it to the desired point. This is done by pa.s.sing a current of electricity through a coil inserted in the system at the point D. This electric reheater consists of a standard "Simplex" coil, so placed in the copper can that the water has a maximum circulation about the heater. The whole device is thoroughly insulated with hair-felt. By connecting the electric reheater with the rheostat on the observer's table, control of the quant.i.ty of electricity pa.s.sing through the coil is readily obtained, and hence it is possible to regulate the temperature of the ingoing water to within a few hundredths of a degree.

The control of the amount of heat brought away from the chamber is made either by (1) increasing the rate of flow or (2) by varying the temperature of the ingoing water. Usually only the second method is necessary. In the older form of apparatus a third method was possible, namely, by varying the area of the absorbing surface of the cooling system inside of the chamber. This last method of regulation, which was used almost exclusively in earlier experiments, called for an elaborate system of s.h.i.+elds which could be raised or lowered at will by the operator outside, thus involving an opening through the chamber which was somewhat difficult to make air-tight and also considerably complicating the mechanism inside the chamber. The more recent method of control by regulating the temperature of the ingoing water by the electric reheater has been much refined and has given excellent service.

_Insulation of water-pipes through the wall._--To insulate the water-pipes as they pa.s.s through the metal walls of the calorimeter and to prevent any cooling effect not measured by the thermometers presented great difficulties. The device employed in the Middletown chamber was relatively simple, but very inaccessible and a source of more or less trouble, namely, a large-sized gla.s.s tube embedded in a large round wooden plug with the annular s.p.a.ce between the gla.s.s and wood filled with wax. An attempt was made in the new calorimeters to secure air insulation by using a large-sized gla.s.s tube, some 15 millimeters internal diameter, and pa.s.sing it through a large rubber stopper, fitting into a bra.s.s ferule soldered between the zinc and copper walls.

(See N, fig. 25.) So far as insulation was concerned, this arrangement was very satisfactory, but unfortunately the gla.s.s tubes break readily and difficulty was constantly experienced. An attempt was next made to subst.i.tute hard-rubber tubing for the gla.s.s tube, but this did not prove to be an efficient insulator. More recently we have used with perfect success a special form of vacuum-jacketed gla.s.s tube, which gives the most satisfactory insulation. However, this system of insulation is impracticable when electric-resistance thermometers are used for recording the water-temperature differences and can be used only when mercurial thermometers exclusively are employed. The electric-resistance thermometers are constructed in such a way, however, as to make negligible any inequalities in the pa.s.sage of heat through the hard-rubber casing. This will be seen in the discussion of these thermometers.

_Measuring the water._--As the water leaves the respiration chamber it pa.s.ses through a valve which allows it to be deflected either into the drain during the preliminary period, or into a small can where the measurements of the rate of flow can readily be made, or into a large tank (G, fig. 14) where the water is weighed. The measurement of the water is made by weight rather than by volume, as it has been found that the weighing may be carried out with great accuracy. The tank, a galvanized-iron ash-can, is provided with a conical top, through an opening in which a funnel is placed. The diagram shows the water leaving the calorimeter and entering the meter through this funnel, but in practice it is adjusted to enter through an opening on the side of the meter. After the valve _f_ is tightly closed the empty can is weighed.

When the experiment proper begins the water-current is deflected so as to run into this can and at the end of an hour the water is deflected into a small can used for measuring the rate of flow. While it is running into this can, the large can G is weighed on platform scales to within 10 grams. After weighing, the water is again deflected into the large can and that collected in the small measuring can is poured into G through the funnel. The can holds about 100 liters of water and consequently from 3 to 8 one-hour periods, depending upon the rate of flow, can be continued without emptying the meter. When it is desired to empty the meter at the end of the period, the water is allowed to flow into the small can, and after weighing G, the valve _f_ is opened. About 4 minutes are required to empty the large can. After this the valve is again closed, the empty can weighed, and the water in the small measuring-can poured into the large can G through the funnel. The scales used are the so-called silk scales and are listed by the manufacturers to weigh 150 kilograms. This form of scales was formerly used in weighing the man inside the chamber.[7]

THERMOMETERS.

In connection with the calorimeter and the accessories, mercurial and electric-resistance thermometers are employed. For measuring the temperature of the water as it enters and leaves the chamber through horizontal tubes, mercurial thermometers are used, and these are supplemented by electric-resistance thermometers which are connected with a special form of recording instrument for permanently recording the temperature differences. For the measurement of the temperatures inside of the calorimeter, two sets of electric-resistance thermometers are used, one of which is a series of open coils of wire suspended in the air of the chamber so as to take up quickly the temperature of the air. The other set consists of resistance coils encased in copper boxes soldered to the copper wall and are designed to indicate the temperature of the copper wall rather than that of the air.

MERCURIAL THERMOMETERS.

The mercurial thermometers used for measuring the temperature differences of the water-current are of special construction and have been calibrated with the greatest accuracy. As the water enters the respiration chamber through a horizontal tube, the thermometers are so constructed and so placed in the horizontal tubes through which the water pa.s.ses that the bulbs of the thermometers lie about in a plane with the copper wall, thus taking the temperature of the water immediately as it enters and as it leaves the chamber. For convenience in reading, the stem of the thermometer is bent at right angles and the graduations are placed on the upright part.

The thermometers are graduated from 0 to 12 C. or from 8 to 20 C.

and each degree is divided into fiftieths. Without the use of a lens it is possible to read accurately to the hundredth of a degree. For calibrating these thermometers a special arrangement is necessary. The standards used consist of well-constructed metastatic thermometers of the Beckmann type, made by C. Richter, of Berlin, and calibrated by the Physikalische Technische Reichsanstalt. Furthermore, a standard thermometer, graduated from 14 to 24 C., also made by Richter and standardized by the Physikalische Technische Reichsanstalt, serves as a basis for securing the absolute temperature. Since, however, on the mercurial thermometers used in the water-current, differences in temperature are required rather than absolute temperatures, it is unnecessary, except in an approximate way, to standardize the thermometers on the basis of absolute temperature. For calibrating the thermometers, an ordinary wooden water-pail is provided with several holes in the side near the bottom. One-hole rubber stoppers are inserted in these holes and through these are placed the bulbs and stems of the different thermometers which are to be calibrated. The upright portion of the stem is held in a vertical position by a clamp. The pail is filled with water, thereby insuring a large ma.s.s of water and slow temperature fluctuations, and the water is stirred by means of an electrically driven turbine stirrer.

The Beckmann thermometers, of which two are used, are so adjusted that they overlap each other and thus allow a range of 8 to 14 C. without resetting. For all temperatures above 14 C., the standard Richter thermometer can be used directly. For temperatures at 8 C. or below, a large funnel filled with cracked ice is placed with the stem dipping into the water. As the ice melts, the cooling effect on the large ma.s.s of water is sufficient to maintain the temperature constant and compensate the heating effect of the surrounding room-air. The thermometers are tapped and read as nearly simultaneously as possible. A number of readings are taken at each point and the average readings used in the calculations. Making due allowance for the corrections on the Beckmann thermometers, the temperature differences can be determined to less than 0.01 C. The data obtained from the calibrations are therefore used for comparison and a table of corrections is prepared for each set of thermometers used. It is especially important that these thermometers be compared among themselves with great accuracy, since as used in the calorimeter the temperature of the ingoing water is measured on one thermometer and the temperature of the outgoing water on another.

Thermometers of this type are extremely fragile. The long angle with an arm some 35 centimeters in length makes it difficult to handle them without breakage, but they are extremely sensitive and accurate and have given great satisfaction. The construction of the bulb is such, however, that the slightest pressure on it raises the column of mercury very perceptibly, and hence it is important in practical use to note the influence of the pressure of the water upon the bulbs and make corrections therefor. The influence of such pressure upon thermometers used in an apparatus of this type was first pointed out by Armsby,[8]

and with high rates of flow, amounting to 1 liter or more per minute, there may be a correction on these thermometers amounting to several hundredths of a degree. We have found that, as installed at present, with a rate of flow of less than 400 cubic centimeters per minute, there is no correction for water pressure.

In installing a thermometer it is of the greatest importance that there be no pressure against the side of the tube through which the thermometer is inserted. The slightest pressure will cause considerable rise in the mercury column. Special precautions must also be taken to insulate the tube through which the water pa.s.ses, as the pa.s.sage of the water along the tube does not insure ordinarily a thorough mixing, and by moving the thermometer bulb from the center of the tube to a point near the edge, the water, which at the edge may be somewhat warmer than at the center, immediately affects the thermometer. By use of the vacuum jacket mentioned above, this warming of the water has been avoided, and in electric-resistance thermometers special precautions are taken not only with regard to the relative position of the bulb of the mercury thermometer and the resistance thermometer, but also with regard to the hard-rubber insulation, to avoid errors of this nature.

ELECTRIC-RESISTANCE THERMOMETERS.

Electric-resistance thermometers are used in connection with the respiration calorimeter for several purposes: first, to determine the fluctuations in the temperature of the air inside the chamber; second, to measure the fluctuations of the temperature of the copper wall of the respiration chamber; third, for determining the variations in body temperature; finally, for recording the differences in temperature of the incoming and outgoing water. While these thermometers are all built on the same principle, their installation is very different, and a word regarding the method of using each is necessary.

AIR THERMOMETERS.