RHEOSTAT FOR HEATING.
The rheostat for heating the air-spaces and the returning air-current about the zinc wall is placed on the observer's table and is indicated in the diagram as R2. 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 S5. The other side of the switch S5 connects directly with the point on the middle of switch S10, 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 R2, are connected with the binding posts on S10, 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 R2, 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 R1. This R1 is connected on one end directly with the 110-volt circuit and the current leaving it passes 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 S9. 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 R1 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, W1 and W2. Bridge W1 is used for the resistance thermometers indicating the temperature of the wall and the air. Bridge W2 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 S1, S2, and S3 allow the use of either the wall, air, or rectal thermometer on either the bed or chair calorimeter at will. Since the bridge W1 is used for measuring the temperature of both the wall and the air, a fourth double-pole switch, S4, is used to connect the air and wall thermometers alternately. The double-contact key, K1, is connected with the bridge W1 and is so arranged that the battery circuit is first made and subsequently the galvanometer circuit. A similar arrangement in K2 controls the connections for the bridge W2.
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 passing through the manganin heating coils in the air-spaces next the zinc wall, a series of resistances is installed connected directly with the rheostat R2 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 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.