Page:The American Cyclopædia (1879) Volume VI.djvu/512

 504 ELECTRICITY means of a circular scale marked upon the cir- cumference of the cylinder, and another suit- ably adjusted at the upper end of the small cylinder from which the wire is suspended. The cap at the top may be turned to increase the force of torsion, which is in proportion to the angle, and bring the balls nearer together ; and it will be found that when the distance has been reduced to one half, the angle of tor- sion will be four times as great. It must be remembered, however, in making the calcula- tion, that the actual distance between the balls is measured by the chord of the arc, and not by the arc itself. With this instrument Coulomb demonstrated the two following laws : 1. The force of attraction and repulsion between two electrified bodies is in the inverse ratio of the squares of their distance. 2. The distance re- maining the same, the force of the attraction and repulsion between two electrified bodies PIG. 4. Coulomb's Torsion Balance Electrometer. is directly as the product of the quantities of electricity with which they are charged. It is a natural conclusion that when a body is elec- trically excited, that is, when it contains free statical electricity, it resides upon the surface, being held there by the resistance of the in- sulating air and the self-repulsion of the fluid ; and such a conclusion is borne out by experi- ment ; but in the case of currents the capacity of a body depends upon the area of its cross section, a hollow wire not being so good a con- ductor as a solid one. If a hollow, insulated globe, fig. 5, having an aperture, is charged with electricity, and a small disk of gilt paper, called a proof plane, is carried by means of an insulating handle into the globe and applied to its interior surface, it will be found upon with- drawal to contain no free electricity. Faraday used a wire-gauze cylinder in place of the globe. It rested upon an insulated metal plate, which being charged of course supplied electricity to the cylinder, but it all resided upon the ex- terior, no trace being found on applying the proof plane to the interior. Coulomb's torsion electrometer may also be used to demonstrate the same condition ; for if a solid and a hol- low sphere of the same diameter be brought FIG. 5. Proof Plane and Hollow Sphere. together, one having free electricity, the charge will be equally divided between them. In consequence of the self-repelling force of the electric fluid which thus causes it to reside upon the exterior of bodies, it will be dis- tributed in equal quantities over the surface of a sphere ; but upon an ellipsoid or an elon- gated cylinder it will be greatest at the ends, and if these be quite acute the tension will be- come so great as to cause it to be discharged in a current. A disk also contains nearly all the charge at the circumference, as may be shown by the use of the proof plane and Cou- lomb's torsion electrometer, substituting the proof plane in place of the gilt ball in the carry- ing rod. An exception to the rule that stati- cal electricity is confined to the exterior of a body is when an insulated electrically excited body is placed in the interior of a hollow con- ductor. In this case the excited body will in- duce upon the interior surface of the conductor a charge of opposite electricity ; and if the con- ductor be insulated, an equal quantity of the same kind will be repelled to the outside. Again, if a conducting body be suspended by a wire connected with the earth inside of a hollow, charged, insulated conductor, it will become charged by induction with electricity opposite to that of the hollow conductor, a part of the charge of the latter being at the same time brought to its inner surface also by induction. If, now, the inner body be insu- lated and lifted out by a silk thread attached to the wire, it will retain its charge of opposite