CONDENSER, the apparatus used in conjunction with an electrometer to increase its sensibility, and render it available for indicating the presence of very feeble electricity. The condensing apparatus consists of two brass plates, which are placed horizontally, the lower one being connected with a metal rod to which gold leaves are attached, and the upper one being provided with an insulating glass handle. These plates are accurately ground, the one to the other, so that when placed upon each other, they touch in every part. Their inner surfaces are covered with a very thin and equable layer of shellac. When an observation is made, the excited body is brought into con tact with the lower plate, and the finger of the observer is laid upon the upper. This being done for a sufficient time, the finger is first removed, and then the excited body, after which the plate is lifted by its handle parallel to the other plate, the gold leaves at the same time diverging under the influence of the electricity left in the lower plate. The same observation might have been made with the positions of the finger and the excited body reversed, but the leaves would then be charged with the opposite electri city to that of the excited body. Reverting to the first case, the electricity to be tested is communicated to the lower plate in small successive charges, which, acting through the thin layer of shellac, induce, as in the Leyden jar, a corresponding charge of the opposite electricity on the lower surface of the upper plate, and send the similar elec tricity of the upper plate through the finger into the ground. Each weak charge of electricity given to the lower plate is not allowed to dissipate, but is kept fixed or bound by the corresponding charge of the opposite electricity which it has induced on the upper plate, so that an accumulation of such charges takes place. As yet, however,
there is no excitement visible in the gold leaves, the electricity so condensed in the plate being capable of acting only in one direction—viz., towards the charge of the upper plate. When, however, the plate is removed, the collected electricity Of the lower plate being no longer restrained to act towards it, immediately extends to the leaves below, and causes a marked divergence. In this manner, electricity of too low a tension to affect immediately the gold leaves can be condensed, so as to possess the power of doing so.
It is found that the efficiency of the C. depends upon the accurate grinding of the plates, the thinness and evenness of the layer of shellac with which their inner surfaces are varnished, the size of the plates, and their parallelism on removal. This last is of the utmost importance; and it is found, where numerical results are wanted, that little dependence can be placed on the parallelism attained by the hand. For more accurate observations, the C. is made quite separate from the electrometer. The plates are in this case attached vertically to two wooden pillars, on which they are insulated. and which slide in a horizontal groove made in the sole of the instrument. The plates, thus guided by the grooves, are made to approach and to retire from each other with their faces parallel. In a C. of this description, no shellac varnish is used, the air between the plates acting as the dielectric in its place. When one of the plates is con nected with the knob of the electrometer, the observation proceeds as already detailed.