What renders the torsion of the metallic wires pe. culiarly adapted for these kind of experiments is its extreme sensibility, by which the slightest variation in the intensity of the attractive or repulsive force produces an enormous change in the angle of tor sion, which we must communicate to the metallic wire in order to balance it. In the experiments, for example, related by Coulomb,—to turn the lever ab a whole circumference, it was only necessary to apply to one of its extremities a force equal to the weight of of a Troy grain.
Another remarkable result presents itself in these experiments. Whatever be the matter of which the balls are composed, provided that they are electrified simultaneously as we have supposed, the electricity di vides-itself between them always in the same ratio. If they are equal in volume, this ratio is that of ty; if they are unequal, the allotment follows a pro portion depending on their diameters; but the che mical nature of the substance has no influence what. ever. This may be proved by the following experi ment : When the moveable ball a, of the balance, has been electrified simultaneously with the fixed ball A, and has receded to a certain distance, where the Tepid sive force maintains it in equilibrio with the force of torsion of the wire; touch the fixed ball A, with an.. other ball of the same diameter insulated at the ex tremity of a small cylinder of gum lac, and with draw it immediately after the contact ; the electri city of the fixed ball will now be reduced exactly one-half, as the moveable ball will indicate by the new distance at which it will place itself. Now, this same reduction of one-half takes place whatever be the chemical nature of the ball with which the con tact is effected. This fact proves, that the electri city is not retained in the balls by a chemical affinity for the rosterial substance of which they are com posed ; and thus furnishes one great character which the theory must admit and represent.
In place of balls, we may equally well employ in these experiments snail 'circular disks of gilt paper ; or any other bodies, whatever be their form, provid ed their dimensions are very small, compared with the distance at which we make them act upon each other ; this condition is, in fact, the only one which is necessary for making all the quantities of electricity which they possess toact together as if they were unit ed in one mangle point. But the spherical form, and
the circular, have peculiar advant. _arising even &con the very manner in which d - disposes itself in the bodies where it is in equilibria as we shall afterwards explain.
This mode of disposition is the first _ to which we must now attend. For, if we • . in deter mining it by experiment, it will furnish us with geo metrical characters, which the theory t to satisfy, and which consequently will throw "- .t upon the conceptions by which electricity may be represented. This determination is still obtained by means of the torsion balance. For this purpose we prepare a small circular disk of gilt pa per, P. (fig. 4.) which we insulate by fix ing it to the extremity ofa very fine cylin der of gum lac, SP; we insulate also, as completely as possible, the body which we mean to study, and then electrify it by communicating a spark drawn from a Leyden phial, or an electrophorus. If we now wish to know the state of any of the points of its surface, we touch this point with the little gilt disk, which we shall call the proof plane, and which, in this experiment, we hold by the extremity S of its insulating suppat, SP. We then carry this plane in the el I balance, where the moveable ball has been previously charged with an electricity of the same kind, and place it for an instant in contact with the fixed ball. We then withdraw it, and the fixed ball being now electrified in the same manner as the moveable one, repels this with a force measured by the angle of tor sion, at which the moveable ball stops. While the little plane and the balls of the balance remain the same, the division of the electricity between the little plane and the moveable ball, follows a constant proportion ' • thus the repulsive force which results, and which drives of the moveable ball, ifproportion al to the quantity of electricity with which the little plane is charged. But, moreover, experience proves that this is proportional to the quantity of electricity which really exists at the point of the body where we made the contact. The repulsive force, therefore, exerted by the fixed ball, is, likewise, proportional to that quantity of electricity, and thus the torsion ob served will give us its measure. By repeating the same proof upon various other points of the body which we submit to experiment, we may determine the manner in which the electricity is distributed throughout.