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 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 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.
[Illustration: 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.]