Part 3 (1/2)
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.