examples of the varied and numerous effects that are produced by the voltaic pile, or, rather, that arise from the continuous reunion of the two electricities; for the pile is in this case only a convenient and powerful means of obtaining this continuous reunion, but it is not the only means. These examples will suffice for the present; for they will already have enabled us to discover, in the effects of the pile, the means of measuring its power, and of constructing apparatus which have been termed Galvanometers, or, better still, Voltameters.

Instruments intended for measuring the Power of the Voltaic Pile, or Voltameters.

The first Voltameter, founded upon the calorific effects of the pile, was contrived by M. Gaspard de la Rive; it consists of a fine platinum wire, stretched vertically, the two extremities of which terminate at pieces

Fig. 19.

of metal, separated from each other by a sufficiently insulating body, such as ivory or ebony (Fig. 19.). The wire, at its lower extremity, is secured to a needle, movable in a vertical plane round one of its extremities, while the other extremity traverses the divisions. of a graduated circle. The point, where the wire and needle are united, is very near its centre of rotation; whence it follows that very small variations in the length of the platinum wire make the needle traverse comparatively large arcs on the scale. The conductors from the poles of the pile are put into communication by means of metal pincers, one with the lower and the other with the upper part of the wire. The latter becomes more or less heated in proportion to the power of the pile, and its dilation is appreciated by the number of degrees traversed by the needle on the scale. Platinum wires of different sizes may be adjusted to the instrument to render it more or less sensible: the choice of these wires depends upon the force of the

apparatus, whose power is by these means to be appreciated. It is easy to determine the expansion that the wire has undergone, and consequently the approximate temperature to which it has been exposed, by means of the sines of the arcs that have been traversed, and by knowing the absolute length of the wire.

A still more sensible voltameter, founded also on the same principle, is that in which the platinum wire, that is to serve

as the channel for the continuous reunion of the two electricities, traverses the ball of an air thermoscope, in such a manner that its two extremities come outside, and thus may each be placed in communication with one of the poles of the pile (Fig. 20.). The wire must be hermetically sealed to the glass at the two points where it traverses the ball. As soon as it is heated it expands the air contained in the ball,-an effect which is immediately made manifest by the rise of the thermoscopic liquid. There are often in the same ball two different and independent wires, the one finer than the other; so that, according to the force of the electricity, an apparatus of greater or less sensibility may be employed.

Fig. 20.

Another, which is no less sensible, and whose indications are

Fig. 21.

more comparable, is a voltameter that I described even before the preceding one had been contrived. It consists in using the helix of a Breguet's metallic thermometer, instead of a platinum wire, to appreciate, by the calorific effects, the energy of the dynamic electricity (Fig. 21.). Breguet's helix is composed of three superposed metallic plates,

so narrow and so thin that the slightest change of temperature makes it twist and untwist by means of the unequal

dilatability of the three metals, platinum, gold, and silver, of which it is formed. The helix is fixed by its upper extremity to a metal support, which is placed in communication with one of the poles of the pile. To its lower extremity, which is free, is soldered a small vertical platinum wire, that is lightly plunged into a capsule filled with mercury, into which the other pole enters by means of a wire. This same free extremity carries the horizontal needle, which, by the number of degrees it traverses on a circular division, indicates the elevation of temperature that the helix has undergone.

These are the three principal forms that have been given to calorific galvanometers, or rather voltameters, which, as their name indicates, are founded on the effects of the heat to which the electricity produced by the pile may give rise.

Voltameters that are founded upon the chemical effects, have for their basis the exact measure of the quantity of gas liberated during a given time (one minute, for example) in the decomposition of water; or, which is still better, the appreciation of the time necessary to liberate a given quantity of gas. It is hence necessary that these gases be collected with care, no portion being allowed to escape, and their volume being accurately estimated, taking into the account the atmospheric pressure and temperature. This is not all: the quantity of gas liberated in a given time, in the decomposition of water, depends not only upon the force of the pile employed, but also on the degree of purity of the water subjected to experiment, and on the nature and size of the wires that are immersed in it, in order to place it in communication with the poles of the apparatus, and also on the distance which separates these wires or plates; so that, in order that experiments may be comparable, it is necessary to employ throughout the same apparatus or apparatus perfectly similar. With regard to the water, distilled water is selected, to which has been added a certain portion of very pure sulphuric acid, in order to render it sufficiently conductible;-about th in volume, or th in weight, is employed. By means of an areometer it is easy to have water always acidulated to the same degree. The metal employed is always platinum, either in wires or in plates.

These wires are placed vertically in a glass, and near to each other; their lower extremity comes out at the bottom of the vessel, so that a communication may be established between them and the poles of the pile. The gases are collected, either in two tubes (Fig. 18.), or in a single one placed equally

Fig. 22. over both wires

Fig. 24.

Fig. 23.

(Fig. 22.). The tubes must be carefully graduated. The gases are also collected in graduated test tubes, placed on the pneumatic trough, and beneath the voltameter (Fig. 23.). When, on the contrary, the liberation of gas is very feeble, the gases are measured by the displacement they produce upon the liquid. With this view, there comes from the lower part of the vessel, wherein the decomposition is carried on, a lateral tube in which the liquid that is driven out by the gas ascends, and is itself lodged in the upper part, which is hermetically sealed (Fig. 24.). With this apparatus it is necessary to take into account the influence which the pressure, exercised by the column of liquid raised in the lateral tube, may have upon the volume of the gas.

All these voltameters, both the chemical and the calorific, are far from being perfect

instruments for there is nothing to prove that their indications are exactly proportional to the intensity of the cause which they are intended to measure; and, moreover, these indications themselves are often variable by reason of circumstances of various kinds, which we shall be called upon to study and to appreciate hereafter.

We may add, that there is one galvanometer, the most perfect of all, which we have not mentioned, because we shall have occasion to devote ourselves to it in a very special manner in the Third Part of this work: it is the one founded upon the property possessed by every conductor that is traversed by an electric current, to deflect a magnetic needle from its natural direction of North and South, when it is placed parallel, either above or below the needle. This instrument has received the name of magnetic galvanometer or multiplier.

Different Forms given to the Voltaic Pile.

The form of a column, which Volta gave to his apparatus, was soon abandoned; it had many inconveniences, without reckoning the very long time that must needs be devoted to putting the apparatus together, whenever it was required to be used. The principal inconvenience was the rapid drying up of the pieces of moistened cloth or pasteboard, whence arose a great diminution in the power of the pile.

To remedy this, it was proposed to substitute for these discs a bed of liquid, which necessarily required that what was a vertical pile must be rendered horizontal, and that each pair should then be composed, not of two circular discs, but of two rectangular plates in contact, and should be cemented one after the other in the grooves of a wooden trough, so as to leave between them vacant spaces or cells, to be filled with liquid (Fig. 25.). It is essential that caution be taken that the plates are carefully cemented against the sides of the trough, that there may be

Fig. 25.