The air thermometers are designed with a special view to taking quickly the temperature of the air. Five thermometers, each having a resistance of not far from 4 ohms, are connected in series and suspended 3.5 centimeters from the wall on hooks inside the chamber. They are surrounded for protection, first, with a perforated metal cylinder, and outside this with a wire guard.

[Ill.u.s.tration: FIG. 15.--Detail of air-resistance thermometer, showing method of mounting and wiring the thermometer. Parts of the wire guard and bra.s.s guard are shown, cut away so that interior structure can be seen.]

The details of construction and method of installation are shown in fig.

15. Four strips of mica are inserted into four slots in a hard maple rod 12.5 centimeters long and 12 millimeters in diameter, and around each strip is wound 5 meters of double silk-covered pure copper wire (wire-gage No. 30). By means of heavy connecting wires, five of these thermometers are connected in series, giving a total resistance of the system of not far from 20 ohms. The thermometer proper is suspended between two hooks by rubber bands and these two hooks are in turn fastened to a wire guard which is attached to threaded rods soldered to the inner surface of the copper wall, thus bringing the center of the thermometer 3.4 centimeters from the copper wall. Two of these thermometers are placed in the dome of the calorimeter immediately over the shoulders of the subject, and the other three are distributed around the sides and front of the chamber. This type of construction gives maximum sensibility to the temperature fluctuations of the air itself and yet insures thorough protection. The two terminals are carried outside of the respiration chamber to the observer's table, where the temperature fluctuations are measured on a Wheatstone bridge.

WALL THERMOMETERS.

The wall thermometers are designed for the purpose of taking the temperature of the copper wall rather than the temperature of the air.

When temperature fluctuations are being experienced inside of the respiration chamber, the air obviously shows temperature fluctuations first, and the copper walls are next affected. Since in making corrections for the hydrothermal equivalent of the apparatus and for changes in the temperature of the apparatus as a whole it is desirable to know the temperature changes of the wall rather than the air, these wall thermometers were installed for this special purpose. In construction they are not unlike the thermometers used in the air, but instead of being surrounded by perforated metal they are encased in copper boxes soldered directly to the wall. Five such thermometers are used in series and, though attached permanently to the wall, they are placed in relatively the same position as the air thermometers. The two terminals are conducted through the metal walls to the observer's table, where variations in resistance are measured. The resistance of the five thermometers is not far from 20 ohms.

ELECTRICAL RECTAL THERMOMETER.

The resistance thermometer used for measuring the temperature of the body of the man is of a somewhat different type, since it is necessary to wind the coil in a compact form, inclose it in a pure silver tube, and connect it with suitable rubber-covered connections, so that it can be inserted deep in the r.e.c.t.u.m. The apparatus has been described in a number of publications.[9] The resistance of this system is also not far from 20 ohms, thus simplifying the use of the apparatus already installed on the observer's table.

ELECTRIC-RESISTANCE THERMOMETERS FOR THE WATER-CURRENT.

The measurement of the temperature differences of the water-current by the electric-resistance thermometer was tried a number of years ago by Rosa,[10] but the results were not invariably satisfactory and in all the subsequent experimenting the resistance thermometer could not be used with satisfaction. More recently, plans were made to incorporate some of the results of the rapidly acc.u.mulating experience in the use of resistance thermometers and consequently an electric-resistance thermometer was devised to meet the conditions of experimentation with the respiration calorimeter by Dr. E. F. Northrup, of the Leeds & Northrup Company, of Philadelphia. The conditions to be met were that the thermometers should take rapidly the temperature of the ingoing and outcoming water and that the fluctuations in temperature difference as measured by the resistance thermometers should be controlled for calibration purposes by the differences in temperature of the mercurial thermometers.

[Ill.u.s.tration: FIG. 16.--Details of resistance thermometers for water-circuit. Upper part of figure shows a sketch of the outside of the hard-rubber case. In lower part is a section showing interior construction. Flattened lead tube wound about central bra.s.s tube contains the resistance wire. A is enlarged part of the case forming a chamber for the mercury bulb. Arrows indicate direction of flow on resistance thermometer for ingoing water.]

For the resistance thermometer, Dr. Northrup has used, instead of copper, pure nickel wire, which has a much higher resistance and thus enables a much greater total resistance to be inclosed in a given s.p.a.ce.

The insulated nickel wire is wound in a flattened spiral and then pa.s.sed through a thin lead tube flattened somewhat. This lead tube is then wound around a central core and the flattened portions attached at such an angle that the water pa.s.sing through the tubes has a tendency to be directed away from the center and against the outer wall, thus insuring a mixing of the water. s.p.a.ce is left for the insertion of the mercurial thermometer. With the thermometer for the ingoing water, it was found necessary to extend the bulb somewhat beyond the resistance coil, so that the water might be thoroughly mixed before reaching the bulb and thus insure a steady temperature. Thus it was found necessary to enlarge the chamber A (fig. 16) somewhat and the tube leading out of the thermometer, so that the bulb of the thermometer itself could be placed almost directly at the opening of the exit tube. Under these conditions perfect mixing of water and constancy of temperature were obtained.

In the case of the thermometer which measured the outcoming water, the difficulty was not so great, as the outcoming water is somewhat nearer the temperature of the chamber, and the water as it leaves the thermometer pa.s.ses first over the mercurial thermometer and then over the resistance thermometer. By means of a long series of tests it was found possible to adjust these resistance thermometers so that the variations in resistance were in direct proportion to the temperature changes noted on the mercurial thermometers. Obviously, these differences in resistance of the two thermometers can be measured directly with the Wheatstone bridge, but, what is more satisfactory, they are measured and recorded directly on a special type of automatic recorder described beyond.

OBSERVER'S TABLE.

The measurements of the temperature of the respiration chamber, of the water-current, and of the body temperature of the man, as well as the heating and cooling of the air-s.p.a.ces about the calorimeter, are all under the control of the physical a.s.sistant. The apparatus for these temperature controls and measurements is all collected compactly on a table, the so-called "observer's table." At this, the physical a.s.sistant sits throughout the experiments. For convenience in observing the mercurial thermometers in the water-current and general inspection of the whole apparatus, this table is placed on an elevated platform, shown in fig. 3. Directly in front of the table the galvanometer is suspended from the ceiling and a black hood extends from the observer's table to the galvanometer itself. On the observer's table proper are all the electrical connections and at the left are the mercurial thermometers for the chair calorimeter. Formerly, when the method of alternately cooling and heating the air-s.p.a.ces was used, the observer was able to open and close the water-valves without leaving the chair.

The observer's table is so arranged electrically as to make possible temperature control and measurement of either of the two calorimeters.

It is impossible, however, for the observer to read the mercurial thermometers in the bed calorimeter without leaving his chair, and likewise he must occasionally alter the cooling water flowing through the outer air-s.p.a.ces by going to the bed calorimeter itself. The installation of the electric-resistance thermometers connected with the temperature recorder does away with the reading of the mercurial thermometers, save for purposes of comparison, and hence it is unnecessary for the a.s.sistant to leave the chair at the observer's table when the bed calorimeter is in use. Likewise the subst.i.tution of the method of continuously cooling somewhat the air-s.p.a.ces and reheating with electricity, mentioned on page 18, does away with the necessity for alternately opening and closing the water-valves of the chair calorimeter placed at the left of the observer's table.

[Ill.u.s.tration: FIG. 17.--Diagram of wiring of observer's table. W_{1}, W_{2}, Wheatstone bridges for resistance thermometers; K_{1}, K_{2}, double contact keys for controlling Wheatstone circuits; S_{1}, S_{2}, S_{3}, double-pole double-throw switches for changing from chair to bed calorimeter; S_{4}, double-pole double-throw switch for changing from wall to air thermometers; G, galvanometer; R_{2}, rheostat. 1, 2, 3, 4, 5, wires connecting with resistance-coils A B D E F and _a b d e f_; S_{2}, 6-point switch for connecting thermal-junction circuits of either bed or chair calorimeter with galvanometer; S_{10}, 10-point double-throw switch for changing heating circuits and thermal-junction circuits to either chair or bed calorimeter; R_{1}, rheostat for controlling electric heaters in ingoing water in calorimeters; S_{8}, double-pole single-throw switch for connecting 110-v. current with connections on table; S_{9}, double-pole single-throw switch for connecting R_{1} with bed calorimeter.]

Of special interest are the electrical connections on the observer's table itself. A diagrammatic representation of the observer's table with its connections is shown in fig. 17. The heavy black outline gives in a general way the outline of the table proper and thus shows a diagrammatic distribution of the parts. The first of the electrical measurements necessary during experiments is that of the thermo-electric effect of the thermal junction systems installed on the calorimeters. To aid in indicating what parts of the zinc wall need cooling or heating, the thermal junction systems are, as has already been described, separated into four sections on the chair calorimeter and three sections on the bed calorimeter; in the first calorimeter, the top, front, rear, and bottom; in the bed calorimeter, the top, sides, and bottom.

CONNECTIONS TO THERMAL-JUNCTION SYSTEMS.

Since heretofore it has been deemed unwise to attempt to use both calorimeters at the same time, the electrical connections are so made that, by means of electrical switches, either calorimeter can be connected to the apparatus on the table.

The thermal-junction measurements are made by a semicircular switch S_{7}. The various points, I, II, III, IV, etc., are connected with the different thermal-junction systems. Thus, by following the wiring diagram, it can be seen that the connections with I run to the different binding-posts of the switch S_{10}, which as a matter of fact is placed beneath the table. This switch S_{10} has three rows of binding-posts.

The center row connects directly with the apparatus on the observer's table, the outer rows connect with either the chair calorimeter or the bed calorimeter. The points marked _a_, _b_, _d_, _e_, _f_, etc., connect with the bed calorimeter and A, B, D, etc., connect with the chair calorimeter. Thus, by connecting the points _g_ and _i_ with the two binding-posts opposite them on the switch S_{10}, it can be seen that this connection leads directly to the point I on the switch S_{7}, and as a matter of fact this gives direct connection with the galvanometer through the key on S_{7}, thus connecting the thermal-junction system on one section of the bed calorimeter between _g_ and _i_ directly with the galvanometer. Similar connections from the other points can readily be followed from the diagram. The points on the switch S_{7} indicated as I, II, III, IV, correspond respectively to the thermal-junction systems on the top, rear, front, and bottom of the chair calorimeter.

By following the wiring diagram of the point V, it will be seen that this will include the connections with the thermal junctions connected in series and thus give a sum total of the electromotive forces in the thermal junctions. The point VI is connected with the thermal-junction system in the air system, indicating the differences in temperature between the ingoing and outgoing air. It will be noted that there are four sections in the chair calorimeter, while in the bed calorimeter there are but three, and hence a special switch S_{3} is installed to insure proper connections when the bed calorimeter is in use.

This system of connecting the thermal junctions in different sections to the galvanometer makes possible a more accurate control of the temperatures in the various parts, and while the algebraic sum of the temperature differences of the parts may equal zero, it is conceivable that there may be a condition in the calorimeter when there is a considerable amount of heat pa.s.sing out through the top, for example, compensated exactly by the heat which pa.s.ses in at the bottom, and while with the top section there would be a large plus deflection on the galvanometer, thus indicating that the air around the zinc wall was too cold and that heat was pa.s.sing out, there would be a corresponding minus deflection on the bottom section, indicating the reverse conditions. The two may exactly balance each other, but it has been found advantageous to consider each section as a unit by itself and to attempt delicate temperature control of each individual unit. This has been made possible by the electrical connections, as shown on the diagram.

RHEOSTAT FOR HEATING.

The rheostat for heating the air-s.p.a.ces and the returning air-current about the zinc wall is placed on the observer's table and is indicated in the diagram as R_{2}. There are five different sets of contact-points, marked 1, 2, 3, 4, and 5. One end of the rheostat is connected directly with the 110-volt circuit through the main switch S_{5}. The other side of the switch S_{5} connects directly with the point on the middle of switch S_{10}, and when this middle point is joined with either _f_ and F, direct connection is insured between all the various heating-circuits on the calorimeter in use. The various numbered points on the rheostat R_{2}, are connected with the binding posts on S_{10}, and each can in turn be connected with _a_ or A, _b_ or B, etc. The heating of the top of the chair calorimeter is controlled by the point 5 on the rheostat R_{2}, the rear by the point 4, the front by the point 3, and the bottom by the point 2. Point 1 is used for heating the air entering the calorimeter by means of an electric lamp placed in the air-pipe, as shown in fig. 25.

The warming of the electrical reheater placed in the water-circuit just before the water enters the calorimeter is done by an electrical current controlled by the resistance R_{1}. This R_{1} is connected on one end directly with the 110-volt circuit and the current leaving it pa.s.ses through the resistance inside the heater in the water-current. The two heaters, one for each calorimeter, are indicated on the diagram above and below the switch S_{9}. The disposition of the switches is such as to make it possible to use alternately the reheaters on either the bed or the chair calorimeter, and the main resistance R_{1} suffices for both.

WHEATSTONE BRIDGES.

For use in measuring the temperature of the air and of the copper wall of the calorimeters, as well as the rectal temperature of the subject, a series of resistance thermometers is employed. These are so connected on the observer's table that they may be brought into connection with two Wheatstone bridges, W_{1} and W_{2}. Bridge W_{1} is used for the resistance thermometers indicating the temperature of the wall and the air. Bridge W_{2} is for the rectal thermometer. Since similar thermometers are inserted in both calorimeters, it is necessary to introduce some switch to connect either set at will and hence the double-throw switches S_{1}, S_{2}, and S_{3} allow the use of either the wall, air, or rectal thermometer on either the bed or chair calorimeter at will. Since the bridge W_{1} is used for measuring the temperature of both the wall and the air, a fourth double-pole switch, S_{4}, is used to connect the air and wall thermometers alternately. The double-contact key, K_{1}, is connected with the bridge W_{1} and is so arranged that the battery circuit is first made and subsequently the galvanometer circuit. A similar arrangement in K_{2} controls the connections for the bridge W_{2}.

GALVANOMETER.

The galvanometer is of the Deprez-d'Arsonval type and is extremely sensitive. The sensitiveness is so great that it is desirable to introduce a resistance of some 500 ohms into the thermal-junction circuits. This is indicated at the top of the diagram near the galvanometer. The maximum sensitiveness of the galvanometer is retained when the connection is made with the Wheatstone bridges. The galvanometer is suspended from the ceiling of the calorimeter laboratory and is free from vibration.

RESISTANCE FOR HEATING COILS.

To vary the current pa.s.sing through the manganin heating coils in the air-s.p.a.ces next the zinc wall, a series of resistances is installed connected directly with the rheostat R_{2} in fig. 17. The details of these resistances and their connection with the rheostat are shown in fig. 18. The rheostat, which is in the right part of the figure, has five sliding contacts, each of which can be connected with ten different points. One end of the rheostat is connected directly with the 110-volt circuit. Beneath the observer's table are fastened the five resistances, which consist of four lamps, each having approximately 200 ohms resistance and then a series of resistance-coils wound on a long strip of asbestos lumber, each section having approximately 15 ohms between the binding-posts. A fuse-wire is inserted in each circuit to protect the chamber from excessive current. Of these resistances, No. 1 is used to heat the lamp in the air-current shown in fig. 25, and consequently it has been found advisable to place permanently a second lamp in series with the first, but outside of the air-pipe, so as to avoid burning out the lamp inside of the air-pipe. The other four resistances, 2, 3, 4, and 5, are connected with the different sections on the two calorimeters. No. 5 corresponds to the top of both calorimeters. No. 4 corresponds to the rear section of the chair calorimeter and to the sides of the bed calorimeter. No. 3 corresponds to the front of the chair calorimeter and is without communication with the bed calorimeter.

No. 2 connects with the bottom of both calorimeters.

It will be seen from the diagrams that each of these resistances can be connected at will with either the bed or the chair calorimeter and at such points as are indicated by the lettering below the numbers. Thus, section 1 can be connected with either the point A or point _a_ on fig.

17 and thus directly control the amount of current pa.s.sing through the corresponding resistance in series with the lamp in the air-current. The sliding contacts at present in use are ill adapted to long-continued usage and will therefore shortly be subst.i.tuted by a more substantial instrument. The form of resistance using small lamps and the resistance wires wound on asbestos lumber has proven very satisfactory and very compact in form.

[Ill.u.s.tration: FIG. 18.--Diagram of rheostat and resistances in series with it. At the right are shown the sliding contacts, and in the center places for lamps used as resistances, and to left the sections of wire resistances.]

TEMPERATURE RECORDER.

The numerous electrical, thermometric, and chemical measurements necessary in the full conduct of an experiment with the respiration calorimeter has often raised the question of the desirability of making at least a portion of these observations more or less automatic. This seems particularly feasible with the observations ordinarily recorded by the physical observer. These observations consist of the reading of the mercurial thermometers indicating the temperatures of the ingoing and outcoming water, records with the electric-resistance thermometers for the temperature of the air and the walls and the body temperatures, and the deflections of the thermo-electric elements.

Numerous plans have been proposed for rendering automatic some of these observations, as well as the control of the heating and cooling of the air-circuits. Obviously, such a record of temperature measurements would have two distinct advantages: (1) in giving an accurate graphic record which would be permanent and in which the influence of the personal equation would be eliminated; (2) while the physical observer at present has much less to do than with the earlier form of apparatus, it would materially lighten his labors and thereby tend to minimize errors in the other observations.

The development of the thread recorder and the photographic registration apparatus in recent years led to the belief that we could employ similar apparatus in connection with our investigations in this laboratory. To this end a number of accurate electrical measuring instruments were purchased, and after a number of tests it was considered feasible to record automatically the temperature differences of the ingoing and outcoming water from the calorimeter. Based upon our preliminary tests, the Leeds & Northrup Company of Philadelphia, whose experience with such problems is very extended, were commissioned to construct an apparatus to meet the requirements of the respiration calorimeter. The conditions to be met by this apparatus were such as to call for a registering recorder that would indicate the differences in temperature between the ingoing and outcoming water to within 0.5 per cent and to record these differences in a permanent ink line on coordinate paper. Furthermore, the apparatus must be installed in a fixed position in the laboratory, and connections should be such as to make it interchangeable with any one of five calorimeters.

After a great deal of preliminary experimenting, in which the Leeds & Northrup Company have most generously interpreted our specifications, they have furnished us with an apparatus which meets to a high degree of satisfaction the conditions imposed. The thermometers themselves have already been discussed. (See page 30.) The recording apparatus consists of three parts: (1) the galvanometer; (2) the creeper or automatic sliding-contact; (3) the clockwork for the forward movement of the roll of coordinate paper and to control the periodic movement of the creeper.

Under ordinary conditions with rest experiments in the chair calorimeter or bed calorimeter, the temperature differences run not far from 2 to 4. Thus, it is seen that if the apparatus is to meet the conditions of the specifications it must measure differences of 2 C. to within 0.01 C. Provision has also been made to extend the measurement of temperature differences with the apparatus so that a difference of 8 can be measured with the same percentage accuracy.

FUNDAMENTAL PRINCIPLE OF THE APPARATUS.

The apparatus depends fundamentally upon the perfect balancing of the two sides of a differential electric circuit. A conventional diagram, fig. 19, gives a schematic outline of the connections. The two galvanometer coils, _fl_ and _fr_, are wound differentially and both coils most carefully balanced so that the two windings have equal temperature coefficients. This is done by inserting a small shunt _y_, parallel with the coil _fl_, and thus the temperature coefficient of _fl_ and _fr_ are made absolutely equal. The two thermometers are indicated as T_{1} and T_{2} and are inserted in the ingoing and outgoing water respectively. A slide-wire resistance is indicated by J, and _r_ is the resistance for the zero adjustment. Ba, Z, and Z_{1} are the battery and its variable series resistances. If T_{1} and T_{2} are exactly of the same temperature, _i. e._, if the temperature difference of the ingoing and outcoming water is zero, the sliding contact _q_ stands at 0 on the slide-wire and thus the resistance of the system from 0 through _fl_, _r_, and T_{1} back to the point C is exactly the same as the resistance of the slide-wire J plus the coil _fr_ plus T_{2} back to the point C. A rise in temperature of T_{2} gives an increase of resistance in the circuit and the sliding contact _q_ moves along the slide-wire toward J maximum until a balance is obtained.