The apparatus employed by Coulomb to determine these important points, is nearly-the same as what we have already described. The only change which he made upon it, was to remove the divided band of pa per z g, (Plate CCXLIV. Fig. 7.) and to substitute in its place a wooden circle, like the horizon of a globe, rest ing on four pillars, and having a diameter nearly double that of the cylinder. This circle is placed in such a manner, that its centre may coincide with the axis of the wire or fibre which suspends the needle, and that the first division on the wooden circle may be in the same line with the fibre of suspension, and the centre of the ball a, when the needle a g is at rest.
1. On the Causc of the Distribution of Electricity.
In order to determine whether electricity was distri buted over conductors by an affinity or electric attrac tion for one body in preference to another, or merely in virtue of a repulsive force, Coulomb instituted the fol lowing experiments.
Exp. 1. Having suspended in the hole m. (Fig. 7.) of the balance, by means of a cylinder of gum lac, a small ball of copper, so as to touch the ball a of the needle as before, he placed the centre of this ball in a straight line with the suspending fibre and the zero of the scale on the wooden circle. The hall a of the needle which touched the copper ball was then distant from the posi tion where the torsion was nothing, by the slim of the semidiameters of the two touching balls. The two balls were then electrified by the insulated pin CAB, as in the former experiments, and the hall a was repelled to the 48th degree upon the scale. The suspending fibre was then twisted 120° by the head of the micrometer, (Fig. 11.) in order to bring back the ball of the needle into contact with the copper ball as before, and as soon as the oscillations of the needle ceased, it stopped at 28°. At this instant Coulomb quickly brought another pith ball of the same size as the copper ball, and insulated with gum lac, into contact with The copper ball, and upon removing the pith ball, the ball a approached the ball of copper. In order to bring it back to its first dis tance of 28°, it was necessary to untwist the fibre, so that the micrometer marked 120° before contact, and only 44° after it.
Exp. 2. In place of the ball of copper, Coulomb now substituted a circle of iron, 10 lines in diameter, suspended by a cylinder of gum lac, and having its ver tical plane passing through the zero on the wooden scale. He then electrified the ball a and the iron disc, as in the preceding experiment, and the ball a was repelled. In order to bring back the ball a into contact with the iron disc, it was necessary to twist the fibre 110°, and the ball a now stopped at 30° from the iron disc. lie
then instantly touched the iron disc with a small circle of paper of the same diameter, and having withdrawn it, he found that, in order to make the ball a stop at 30°, it was necessary to untwist the fibre, or reduce the tor sion to 40°.
Now, in the first of these experiments, the copper ball having, before its contact with the pith ball, repelled the ball a to 28°, while the micrometer marked 120°, the force of torsion was then But after its contact with the pith ball, it repelled the ball a only to 28°, while the micrometer indicated only 44°, so that the total force of torsion, equal to the repulsive force of the two balls, was only 44°+28=72. A minute, how eve•, had elapsed between the two observations, and as the -dissipation was then -1th per minute, the total force of torsion must be corrected by of 72° or le nearly, so that the corrected force of torsion will be 73e, which differs only from 74', the half of 148°, the former total force of torsion which measured the electric repulsion before the electricity of the cop per ball was communicated to the ball of elder : Since the distance of the balls, therefore, was in both cases the same, and since the action is directly as the densi ties of the fluid, and inversely as the square of the distance, it follows, that the pith ball has received exact ly one half of the electricity of the ball of copper, and that the ball of copper has no more affinity or elective attraction for the electric matter than the pith ball.
In the second experiment, also, the electricity was distributed equally between the iron disc and the disc of paper, and by tiling various other substances, and repeating the experiments by means of a large torsion balance with globes of five or six inches in diameter, CouloMb always obtained the same result.
In repeating the preceding experiments, we must take care to allow the pith and the copper balls to re main a short time in contact; for when one of the sub stances is an imperfect conductor, several seconds elapse before it is able to deprive the other of one half of its electricity. The time in which this distribution takes place, depends not only on the conducting power of the two bodies, but on their relative extent, and on the man ner in which the contact is effected. In bringing into contact two circular discs, as in the second experiment, we must take care that they touch one another symme trically, or that the sum face of the one is in the same plane as the surface of the other. When this precau tion is not taken, the electric matter will be distributed unequally between the two discs.