of insulating walls, and perhaps reducing radiation to the
minimum, but for extreme accuracy in calorimetric investigations it is
necessary to insure the absence of radiation, and hence we have retained
the ingenious device of Rosa, by which an attempt is made arbitrarily to
alter the temperature of the zinc wall so that it always follows any
fluctuations in the temperature of the copper wall. To this end it is
necessary to know _first_ that there is a temperature difference between
zinc and copper and, _second_, to have some method for controlling the
temperature of the zinc. Leaving for a moment the question of measuring
the temperature differences between zinc and copper, we can consider
here the methods for controlling the temperature of the zinc wall.

If it is found necessary to warm the zinc wall, a current of electricity
is passed through the resistance wire W, fig. 12. This wire is
maintained approximately in the middle of the air-space between the zinc
wall and hair-felt by winding it around an ordinary porcelain insulator
F, held in position by a threaded rod screwed into a brass disk soldered
to the zinc wall. A nut on the end of the threaded rod holds the
insulator in position. Much difficulty was had in securing a resistance
wire that would at the same time furnish reasonably high resistance and
would not crystallize or become brittle and would not rust. At present
the best results have been obtained by using enameled manganin wire. The
wire used is No. 28 American wire-gage and has resistance of
approximately 1.54 ohms per foot. The total amount of wire used in any
one circuit is equal to a resistance of approximately 92 ohms. This
method of warming the air-space leaves very little to be desired. It can
be instantaneously applied and can be regulated with the greatest ease
and with the greatest degree of refinement.

If, on the other hand, it becomes necessary to cool the air-space next
to the zinc and in turn cool the zinc, we must resort to the use of cold
water, which is allowed to flow through the pipe C suspended in the
air-space between the zinc and hair-felt at approximately the same
distance as is the heating wire. The support of these pipes is
accomplished by placing them in brass hangers G, soldered to the zinc
and provided with an opening in which the pipe rests.

In the early experimenting, it was found impracticable to use piping of
very small size, as otherwise stoppage as a result of sedim