ts 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