[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