As can be seen from these drawings the whole construction of the
apparatus is more or less of the refrigerator type, _i. e._, there is
little opportunity for radiation or conduction of heat. Such a
construction could be multiplied a number of times, giving a greater
number 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 sediment could
easily occur. The pipe found best adapted to the purpose was the
so-called standard one-eighth inch brass pipe with an actual internal
diameter of 7 millimeters. The opening of a valve allowed cold water to
flow through this pipe and the considerable mass of water passing
through produced a very noticeable cooling effect. In the attempt to
minimize the cooling effect of the mass of water remaining in the pipe,
provision was made to allow water to drain out of this pipe a few
moments after the valve was closed by a system of check-valves. In
building the new apparatus, use was made of the compressed-air service
in the laboratory to remove the large mass of cold water in the pipe. As
soon as the water-valve was closed and the air-cock opened, the
compressed air blew all of the water out of the tube.
[Illustration: FIG. 13.--Detail of drop-eight feed-valve and arrangement
of outside cooling circuit. The water enters at A, and the flow is
regulated by the needle-valve at left-hand side. Rate of flow can be
seen at end of exit tube just above the union. The water flows out at C
and compressed air is admitted at B, regulated by the pet-cock.]
The best results have been obtained, however, with an entirely new
principle, namely, a few drops of water are continually allowed to pass
into the pipe, together with a steady stream of compressed air. This
cold water is forcibly blown through the pipe, thus cooling to an amount
regulated by the amount of water admitted. Furthermore, the relatively
dry air evaporates some of the water, thereby producing a somewhat
greater cooling effect. By adjusting the flow of water through the pipe
a continuous cooling effect of mild degree may be obtained. While
formerly the air in the space next the zinc wall was either cooled or
heated alternately by opening the water-valve or by passing a current
through the heating coil, at present it is found much more advantageous
to allow a slow flow of air and water through the pipes continuously,
thus having the air-space normally somewhat cooler than is desired. The
effect of this cooling, therefore, is then counterbalanced by passing an
electric current of varying strength through the heating wire. By this
manipulation it is unnecessary that the observer manipulate more than
one instrument, namely, the rheostat, while formerly he had to
manipulate valves, compressed-air cocks, and rheostat. The arrangement
for providing for the amount of compressed air and water is shown in
fig. 13, in which it is seen that a small drop-sight feed-water valve is
attached to the pipe C leading into the dead air-space surrounding the
calorimeter chamber. Compressed air enters at B and the amount entering
can be regulated by the pet-cock. The amount of water admitted is
readily observed by the sight feed-valve. When once adjusted this form
of apparatus produces a relatively constant cooling effect and
facilitates greatly the manipulation of the calorimetric apparatus as a
whole.