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

Section 1 Can Be Connected With Either the Point a or Point _a_ on Fig.

17 and thus directly control the amount of current passing 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 substituted 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.

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.

Provision is made for automatically moving the contact q by electrical means and thus the complete balance of the two differential circuits is maintained constant from second to second. As the contact q is moved, it carries with it a stylographic pen which travels in a straight line over a regularly moving roll of coordinate paper, thus producing a permanently recorded curve indicating the temperature differences. The slide-wire J is calibrated so that any inequalities in the temperature coefficient of the thermometer wires are equalized and also so that any unit-length on the slide-wire taken at any point along the temperature scale represents a resistance equal to the resistance change in the thermometer for that particular change in temperature. With the varying conditions to be met with in this apparatus, it is necessary that varying values should be assigned at times to J and to r. This necessitates the use of shunts, and the recording range of the instrument can be easily varied by simple shunting, i. e., by changing the resistance value of J and r, providing these resistances unshunted have a value which takes care of the highest obtained temperature variations.

Fig. 19 shows the differential circuit complete with all its shunts. S is a fixed shunt to obtain a range on J; S' is a variable shunt to permit very slight variations of J within the range to correct errors due to changing of the initial temperatures of the thermometers; y is a permanent shunt across the galvanometer coil fl, to make the temperature coefficients of fl and fr absolutely equal; Z is the variable resistance in the battery-circuit to keep the current constant; r is a permanent resistance to fix the zero on varying ranges; S'' plus S{1} constitutes a variable shunt to permit slight variations of r to finally adjust 0 after S' is fixed and t is a permanent shunt across the thermometer T{1} to make the temperature coefficient of T{1} equal to that of T{2}.

The apparatus can be used for measuring temperature differences from 0° to 4° or from 0° to 8°. When on the 0° to 8° range, the shunt S is open-circuited and the shunt S' alone used. The value of S, then, is predetermined so as to affect the value of the wire J and thus halve its influence in maintaining the balance. Similarly, when the lower range, i. e., from 0° to 4°, is used, the resistance r is employed, and when the higher range is used another value to r must be given by using a plug resistance-box, in the use of which the resistance r is doubled.

The resistance S'' and S{1} are combined in a slide-wire resistance-box and are used to change the value of the whole apparatus when there are marked changes in the position of the thermometric scale. Thus, if the ingoing water is at 2° C. and the outcoming water at 5° C. in one instance, and in another instance the ingoing water is 13° and the outgoing water is 15°, a slight alteration in the value of S{1}, and also of S', is necessary in order to have the apparatus draw a curve to represent truly the temperature differences. These slight alterations are determined beforehand by careful tests and the exact value of the resistances in S' and in S_{1} are permanently recorded for subsequent use.

THE GALVANOMETER.

The galvanometer is of the Deprez-d'Arsonval type and has a particularly powerful magnetic field, in which a double coil swings suspended similar to the marine galvanometer coils. This coil is protected from vibrations by an anti-vibration tube A, fig. 20, and carries a pointer P which acts to select the direction of movement of the recording apparatus, the movable contact point q, fig. 19. In front of this galvanometer coil and inclosed in the same air-tight metal case is the plunger contact Pl, fig. 21. The galvanometer pointer P swings freely below the silver contacts S{1} and S{2}, just clearing the ivory insulator i. The magnet plunger makes a contact depending upon the adjustment of a clock at intervals of 2 seconds. So long as both galvanometer coils are influenced by exactly the same strength of current, the pointer will stand in line with and immediately below i and no current passes through the recording apparatus. Any disturbance of the electrical equilibrium causes the pointer P to swing either toward S{1} or S{2}, thus completing the circuit at either the right hand or the left hand, at intervals of 2 seconds. The movement of the pointer away from its normal position exactly beneath i to either S{1} on the left hand or S{2} on the right, results from an inequality in the current flowing through the two coils in the galvanometer. The difference in the two currents passing through these coils is caused by a change in temperatures of the two thermometers in the water circuit.

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