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 R_{2}. 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
S_{5}. The other side of the switch S_{5} connects directly with the
point on the middle of switch S_{10}, 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 R_{2}, are connected with the binding
posts on S_{10}, 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 R_{2}, 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 R_{1}. This R_{1} 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 S_{9}. 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 R_{1} suffices for
both.