The Thomson pile, which is employed with success for putting in action
the siphon recorder, and which is utilized in a certain number of cases
in which an energetic and constant current is needed, is made in two
forms. We shall describe first the one used for demonstration. Each
element of this (Fig. 1) consists of a disk of copper placed at the
bottom of a cylindrical glass vessel, and of a piece of zinc in the form
of a grating placed at the upper part, near the surface of the solution.
A glass tube is placed vertically in the solution, its lower extremity
resting on the copper. Into this tube are thrown some crystals of
sulphate of copper, which dissolve in the liquid, and form a solution of
a greater density than that of the zinc alone, and which, consequently,
cannot reach the zinc by diffusion. In order to retard the phenomenon of
diffusion, a glass siphon containing a cotton wick is placed with one of
its extremities midway between the copper and zinc, and the other in
a vessel outside the element, so that the liquid is sucked up slowly
nearly to its center. The liquid is replaced by adding from the top
either water or a weak solution of sulphate of zinc.
[Illustration: FIG. 1.--THE THOMSON PILE.(Type for demonstration.)]
The greater part of the sulphate of copper that rises through the liquid
by diffusion is carried off by the siphon before reaching the zinc, the
latter being thus surrounded with an almost pure solution of sulphate
of copper having a slow motion from top to bottom. This renewal of the
liquid is so much the more necessary in that the saturated solution of
sulphate of copper has a density of 1.166, and the sulphate of zinc
one of 1.445, There would occur, then, a mixture through inversion of
densities if the solution were allowed to reach a too great amount of
saturation, did not the siphon prevent such a phenomenon by sucking up
the liquid into the part where the mixture tends to take place. The
chemical action that produces the current is identical with that of the
Daniell element.
In its application, this pile is considerably modified in form
and arrangement. Each element (Fig 2) consists of a flat wooden
hopper-shaped trough, about fifty centimeters square, lined with sheet
lead to make it impervious. The bottom is covered with a sheet of copper
and above this there is a zinc grate formed of closely set bars that
allow the liquid to circulate. This grate is provided with a rim which
serves to support a second similar element, and the latter a third,
and soon until there are ten of the elements superposed to form series
mounted for tension. The weight of the elements is sufficient to secure
a proper contact between the zinc and copper of the elements placed
beneath them, such contact being established by means of a band of
copper cut out of the sheet itself, and bent over the trough.
[Illustration: FIG. 2.--THE THOMSON PILE. (Siphon Recorder Type.)]
On account of the large dimensions of the elements, and the proximity
of the two metals, a pile is obtained whose internal resistance is very
feeble, it being always less than a tenth of an ohm when the pile is
in a good state, and the electromotive force being that of the Daniell
element--about 1 08 volts.
Sometimes the zinc is covered with a sheet of parchment which more
thoroughly prevents a mixture of the liquids and a deposit of copper on
the zinc. But such a precaution is not indispensable, if care be taken
to keep up the pile by taking out some of the solution of sulphate of
zinc every day, and adding sulphate of copper in crystals. If the pile
is to remain idle for some time, it is better to put it on a short
circuit in order to use up all the sulphate of copper, the disappearance
of which will be ascertained by the loss of blue color in the liquid. In
current service, on the contrary, a disappearance of the blue color will
indicate an insufficiency of the sulphate, and will be followed by a
considerable reduction in the effects produced by the pile.
The great power of this pile, and its constancy, when it is properly
kept up, constitute features that are indispensable for the proper
working of the siphon recorder--the application for which it was more
especially designed.
This apparatus has been also employed under some circumstances for
producing an electric light and charging accumulators; but such
applications are without economic interest, seeing the enormous
consumption of sulphate of copper during the operation of the pile.
The use of the apparatus is only truly effective in cases where it is
necessary to have, before everything else, an energetic and exceedingly
constant current.--_La Nature_.
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