At the period when every combination was at tempted to form an electromotive apparatus entirely composed of dry substances, and consequently unal terable, Ritter of Munich discovered one, which, without the power of developing electricity by its own action, is yet susceptible of being charged by the voltaic pile, so as to acquire from it for a time all its properties. These have been named the se condary piles of Ritter.
To form a just and precise idea of this arrange ment, we must take notice of an observation previ ously made by Mr Hermann of Berlin, on the im perfect conductibility of vegetable substances soaked in water.
If we insulate an electrical column, of which the su perior pole is vitreous and the inferior one resinous, and make these two poles communicate by an im perfect conductor ; such, for example, in the case of these small quantities of each as a slip of pa per moistened in pure water ' • half of this slip will take the electricity of the pole with which it communicates, the superior part becoming vitreous, and the inferior resinous. This phenomenon is an evident consequence of the laws by which electricity is distributed among bodies that transmit it impel.• reedy.
Conceive now this imperfect conductor removed from the pile by some insulating body, as, for ex ample, a stick of glass, and let it be so suspended in a dry atmosphere ; then the equilibrium will not be instantly restored between its two extremities, but they will remain during some time vitreous and resin ous as when they communicated with the two poles of the pile. These differences will diminish, by de grees, as the contrary electricities are recombined, and in a short time, their actions being neutralized, will become altogether insensible.
Such is precisely the case with the fundamental experiment of Ritter ; only that for the moistened slip of paper, he substitutes a column composed of discs of copper and moistened cards intermixed. This column is incapable of itself of setting the electricity in motion ; at least, if we suppose each species of its elements to be homogeneous in regard to each other. But it will charge itself by a communication with the pile, in the same manner as the slip of moistened paper above mentioned. There is yet an essential difference in the two results. Electricity, it ap pears, when weak, has some difficulty in passing from one surface to another. This seems, at least,
the result of Ritter's experiments ; and perhaps such resistance is itself produced by the impercep tible stratum of non-conducting air which adheres to the surface of all bodies. The electricity then introduced into the column constructed with a sin gle metal, passes, in like manner, with difficulty from each piece of metal to the preceding moist ened card which is contiguous to it, and this ob stacle increases in proportion as the alternations are more numerous. Such a pile, once charged, must therefore lose its electricity very slowly when there is no direct communication between its two poles. But if we form this communication by a good con ductor, the escape of the two electricities, and their union together being very quickly effected, will pro duce a discharge which, in the same manner as in the leaden jar, will operate by an instantaneous shack. To this effect will succeed a new state of equilibrium, in which the repulsive forces of the dif ferent plates will be diminished in proportion to the quantity of electricity instantaneously neutralized. The discharges then must be repeated with di minished effect as we repeat the contacts, but will soon cease to be sensible, in consequence even of the equality of the charge which they tend to re establish throughout the different parts of the apps, ratus. In a word, the action of this column resem bles that which would successively take place with a more or less perfect conductor, according as its two extremities should communicate or not with each other.
As to the distribution of the electricity throughout the pile, it must be such, that the repulsive force of that portion which is at the surface of each plate, com bined with the resistance of the adjoining surfaces, shall be in equilibrio with the united actions of all the rest of the plates ; consequently, if we suppose the number of elements to be odd, and all the appa ratus insulated, the quantities of electricity will go on diminishing from the two extremities where they are equal and of a contrary nature, as in the pri mitive• pile, towards the centre where they vanish. But if the apparatus communicates with the ground at its base, the electricity will go on increasing throughout the whole extent of the column, from this base, where it will be nothing, to the sum mit, where it will be equal to that of the primitive pile.