Since Poggendorff in 1842 thought of substituting in the Bunsen battery a
solution of bichromate of potash and sulphuric acid for nitric acid, and
of thus making a single liquid pile of it, in suppressing the porous
vessel, his idea has been taken up a considerable number of times. Some
rediscovered it simply, while others, who were better posted in regard to
the work of their predecessors, took Poggendorff's pile as he conceived
it, and, considering the future that was in store for it, thought only of
modifying it in order to render it better. Among these, Mr. Grenet was one
of the first to present the bichromate of potash pile under a truly
practical form. As long ago as 1856, in fact, he gave it the form that is
still in use, and that is known as the bottle pile. Thus constructed, this
pile, as is well known, presents a feeble internal resistance, and a
greater electro-motive power than the Bunsen element. Unfortunately, its
energy rapidly decreases, and the alteration of the liquid, as well as the
large deposit of oxide of chromium that occurs on the positive electrode,
prevents its being employed in experiments of quite long duration. Mr.
Grenet, it is true, obviated these two defects by first renewing the
liquid slowly and continuously, and causing a current of air to bubble up
in the pile so as to detach the oxide of chromium in measure as the
deposit formed. Thus improved, the bichromate pile was employed on a large
scale in the lighting of the Comptoir d'Escompte. In an extensive
application like this latter, the use of compressed air for renewing the
liquid can be easily adapted to the bichromate pile, as the number of
elements is great enough to allow of the putting in of all the piping
necessary; but when it is only desired to use this pile for laboratory
purposes, and when there is need of but a small number of elements, it is
impossible to adopt Mr. Grenet's elements in the form required by an
electric lighting installation. It becomes absolutely necessary, then, to
come back to a simpler form, and attempt at the same time to obviate the
defects which are inherent to its very principle. In accordance with this
idea, it will be well to point out the arrangement adopted by Mr. Courtot
for his bichromate of potash piles--an arrangement that is very simple,
but, sufficiently well worked out to render the use of it convenient in a
laboratory.
[Illustration: Fig. 1.--COURTOT'S ARRANGEMENT OF THE BICHROMATE PILE.]
Fig. 1 gives the most elementary form. It consists of an earthen vessel
into which dip four carbon plates connected with each other by a copper
ring which carries one of the terminals. In the center there is a
cylindrical porous vessel that contains a very dilute and feebly
acidulated solution of bichromate of potash into which dips a prism of
zinc, which may be lifted by means of a rod when the pile ceases to
operate. It is true that the presence of the porous vessel in the
bichromate of potash element increases the internal resistance, but, as an
offset, although it decreases the discharge, it secures constancy and
quite a long duration for it.
[Illustration: Fig. 2.--COURTOT'S ARRANGEMENT OF THE BICHROMATE PILE.]
The elements thus constituted may be grouped, to the number of six, in a
frame analogous to that shown in the engraving, and, sum total, form a
small sized battery adapted to the current experiments of the laboratory,
and capable of supplying two small four volt lamps for ten or twelve
hours. We have had occasion to make use of these elements for the
graduation of galvanometers, and, after ascertaining the constancy of the
discharge, have found that the internal resistance of each couple is
nearly 0.175 ohm, with an electro-motive force of two volts. As may be
seen, these elements should, in general, all be mounted for tension, as
they are in the figure, inasmuch as the mobility of the zincs permits,
according to circumstances, of employing a variable number of them without
changing anything. Moreover, with zincs amalgamated in a special manner,
the attack is imperceptible, and the work in open circuit need scarcely to
be taken into consideration.
Yet, despite the qualities inherent to the arrangement that we have just
described, that defect common to all bichromate of potash piles--the
deposit of oxide of chromium upon the carbon--is not here avoided. It
occurs quite slowly, to be sure, but it does occur, and, from this point
of view, the arrangement shown in Fig. 2 is preferable. The elements here
are composed of prismatic porcelain vessels containing, as before, the
solution and porous vessel.
[Illustration: Fig. 3.--COURTOT'S ARRANGEMENT OF THE BICHROMATE PILE.]
The whole is covered with a sheet of ebonite connected with the zinc and
the two carbon plates in such a way that when the pile is not in operation
the whole can be lifted from the liquid. Under such circumstances the
deposit of oxide is notably diminished, and the duration of the discharge
is consequently greatly increased.
Fig. 3 shows the details of a windlass that permits of lifting, according
to circumstances, all the elements of the same trough or only a part of
them. To effect this, the drum around which the chain winds that carries
the carbons is mounted upon a sleeve fixed upon the axle. This latter is
actuated by a winch; and a ratchet wheel, R, joined to a click which is
actuated by a spiral spring, prevents the ebonite plates from falling back
when it is desired to place the bolt under the button, B, of the spring.
When it is desired to put an element out of the circuit, it is only
necessary to act with the finger upon the extremity of the lever, D. Under
the action of the latter, the piece, _s_, which carries a groove for the
passage of the screws that fix it to the upper cross-piece, takes on a
longitudinal motion and consequently gears with the drum through the
toothed sleeve, E. When an experiment is finished the zinc may thus be
lifted from the liquid, and the deposit of oxide be prevented from forming
upon the carbon. As may be seen, the arrangements which we have just
described exhibit nothing that is particularly original. The windlasses
used for removing the elements from a pile when the circuit is open have
been employed for a long time; the bichromate pile is itself old, and, as
we said in the beginning, it has been modified in its details a number of
times. In spite of this, we have thought it well to point out the mode of
construction adopted by Mr. Courtot, since, owing to the simplicity of the
arrangements, it renders convenient and easily manageable a pile of very
great constancy that may be utilized for supplying incandescent lamps, as
well as for the most varied experiments of the laboratory.--_La Lumiere
Electrique._
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