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.
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.