2. On the Superficial Distribution of Electricity.
We have hitherto seen that in the communication and distribution of the electric matter, every part of the surface of an electrified body is charged with electricity ; we have now to inquire whether this electricity per vades the whole mass of the body, or is merely distri buted over its surface. The determination of this point is owing likewise to Coulomb. As a more delicate instrument was necessary for these experiments, he made the following change in his apparatus : Having formed a fibre of gum lac, about 10 or 12 lines long, and nearly as thick as a strong hair, one of its extremi ties was attached to the top of a small pin without a head, suspended to a silk fibre, such as comes from the silk worm; the other extremity of the gum lac fibre being fixed to a small circle of tinsel about two lines in diameter. When this electrometer is suspended in a cylinder of glass, its sensibility is so great, that a force equal to a sixty thousandth pats of a grain is sufficient to repel the ball and the needle to a distance of more than 90 degrees. IL.ving communicated a small de gree of electricity to the circle of tinsel, Coulomb sus pended it in a glass cylinder, so as not to be affected by currents of air. Ile then took a solid cylinder of wood, four inches in diameter, and pierced with several holes, four lines broad and four lines deep, and having placed this cylinder upon an insulating stand, he gave it SCA c ral electric sparks, either by means of a Leyden jar, or the metallic plate of an electrophorus. He next insu lated, at the extremity of a small cylinder of gum lac a line in diameter, a circle of gilt paper a line and a half in diameter, and with this apparatus he made the fol lowing experiments: Exp. 1. The tinsel of the electrometer being electri fied, as we have already stated, he touched the surface of the electrified wooden cylinder by the circle of gilt paper, and upon presenting it to the electrometer, the tinsel was repelled with force.
Exp. 2. He then introduced the circle of gilt paper into one of the holes of the cylinder, so as to make it touch the bottom of the hole, and upon presenting it to the electrometer no signs of electricity were exhibited.
Now, in the first experiment, the small circle of gilt paper, which was only the eighteenth part of a line in thickness, became a part of the surface of the cylinder when it was made to touch it, and consequently received a quantity of electricity equal to that which was con tained in a part of the surface, equal to that of the small circle. The small circle %vas not only charged
with a quantity of electricity perceptible to the small electrometer, but capable of being exactly measured by it.
In the second experiment, on the contrary, when the small circle was placed at the bottom of one of the holes, four lines below the surface, and twenty lines from the axis of the cylinder, and when it was taken out of the hole, so as not to touch its sides, it exhibited no marks of electricity ; and hence it follows that there was no electric matter in the interior of the cylinder, even at the small depth of four lines. On some occa sions, the circle of gilt paper exhibited signs of a weak electricity, opposite to that of the cylinder, an effect which Coulomb ascribes to a small degree of electricity received by the gum lac, from its being within the at mosphere of the electrified cylinder. In order to prove that this opposite electricity existed in the gum lac, and not in the circle of gilt paper, he touched the gilt circle, and was not able to destroy its electricity. The oppo site electricity of the gilt circle is, however, always very feeble when the gum lac is pure, and when the weather is not very damp.
3. On the Distribution of Electricity between two con ducting Bodies in contact.
In the experiments of Coulomb on this subject, he mund it necessary to employ a torsion balance of a lar ger size than that which is represented in Plate CCXLV. and of a different form. Time two balances which he used, are represented in Plate CCXLV. Fig. 1, and 2. In Fig. 1, All is a square box, formed by lour plates of glass, 2 feet long and 15 or 16 inches high, which must be placed upon a table, very dry, and coated with a non-conducting varnish. This square box is covered with glass, so as to leave an opening for introducing the globe a, placed at•the end of a small cylinder a c of gum lac, which is terminated upwards in a small cylinder of baked wood, coated with gum lac, and passing perpen dicularly through a hole in the piece of wood c d, in which it is stopped by a screw e. The vertical tube in from 12 to 15 inches high, is made of glass, and is sup ported by a frame p q r s, carrying a semicircle t, 0, up, about four feet in diameter, having its centre coincident with the vertical line in n, and divided into two quadrants of 90° each, beginning at O. The suspending fibre me n carries the piece u v, to which is attached a thread of gum lac u b, terminated in b by a small disc of gilt paper.