Were the pile fitted up in a contrary order, the zinc communicating with the ground, the free elec tricity at its summit would be resinous, and the charge of the condenser would be equal to the pre ceding, but also resinous : all these results are con formable to observation.
As the electricity of the column accumulates in the condenser, so it may spread itself in the interior of a Leyden jar, or of an electrical battery, the ex terior part of which communicates with the ground; and, as the pile, in proportion as it discharges, re charges itself again, from the ground, the battery, put in communication with its insulated pole, will charge itself equally well, whatever be its capacity, until the repulsive force of its free electricity comes to balance that which exists at the pole with which it is in contact. If we then withdraw the battery, it will give a shock corresponding to this repulsive force ; and this also is confirmed by experiment.
In order that the action of the condenser on the ap paratus, either of the pile or the troughs, may be regu lar, constant, and as powerful as possible, the greatest care must be taken to form, between its plates and the poles of the apparatus, the most perfect communi cations. For the quantities of free electricity being excessively minute, the least obstacle is sufficient to stop them, or, at least, considerably to retard their propagation ; and in that case the condenser will take much less electricity than it would have done if the communications had been perfect. It is even much worse, if the mode of communication is itself vari able ; as when we hold the condenser in the hand, and merely place on the summit of the pile the ex tremity of a metallic wire fixed to its collecting plate. In this case, if we apply it several times in succes sion to the same pile, the quantities of electricity with which it is charged may vary in an instant from one to three, or even four times greater or less, in place of that perfect degree of equality which we would obtain'with a more uniform mode of commu nication, and which is indeed absolutely necessary to discover the state of the pile, and to measure it with exactness.
The following is the arrangement of the appara tus, which, after many trials, we have found the most commodious. On a solid table, fix with screws a parallelopiped of wood AB, fig. 11, covered with tin foil. The extremity A of this parallelopiped carries a cone of metal, truncated, well polished, and on which the pile is laid. The other extre mity B carries an upright and moveable stick of me tal TT, terminated by a metal plate, to which the foot of the condenser is firmly fixed by a metal screw. This instrument may then be adjusted to the height of the pile, with which the experiments are made without altering the proper condition of communications. The plates made use of are all of
the same dimensions, and each plate of sine is strong ly attached, but not soldered, to the corresponding plate of copper, so that the contact is in this manner always completely established between them. We have only, then, to dispose the pairs above each other i • and when the plates are new, those pairs may be reckoned the same. As they are also perfectly plane, the pile may be easily enough erect ed by placing them above each other, without any lateral supports, and by this method we also avoid that kind of communication between the poles of the pile, which arises, to the great injury of the appara tus, from the imperfect insulation of these supports.
Lastly, to establish constantly, and in the same uniform manner, the contact of the condenser with the top of the pile, place on this a little cup of iron, filled with mercury, and whose inferior surface is perfectly flat ; and let us suppose, that the extre mity of the flexible stick of the condenser is made with iron, as the little cup itself; then, when the in strument is adjusted to the height of the pile, we have only to immerse the end of this stick into the mercury, by means of a tube of varnished glass, and abandoning the stick to its own elasticity, we are certain of obtaining a contact as equal and instanta neous as possible; which may also be still farther prolonged, to observe, if desired, the influence of time on the charge of the condenser. When the stick is taken out of the mercury, we raise the collecting plate quite parallel to itself, and touch it with the fixed insulated ball of the electrical balance. (See ELEC TRICITY, Supplement.) We replace this in its glass case, and the moveable disk of the balance which may be supposed in its natural state, touching it, is , immedioely repelled to a certain distance, which we can observe; or still better, we may turn the thread of suspension, until the disk is brought to a fixed dis tance from the sphere. Whichever way we adopt, as the disk will become electrified by the contact at the expense of the ball, the angle of torsion will mea sure the square of the quantity of electricity com municated to the ball by the condenser, and to this last by the pile, and we shall then be enabled to esti mate this quantity very exactly. In using this me thod, we always obtain, by a series of consecutive ex periments, results that agree perfectly with each other; which is far from taking place, when we neglect the precautions to ensure the perfection and the identity of the contact of the condenser : it is evident, besides, that the same disposition of the condenser is equally applicable to the apparatus of troughs.