Exh. 5. If a man, insulated as before, is electrified, the electricity will be rapidly discharged when he holds out any pointed body, and in the dark the light will be seen streaming from the point.
Exit. 6. if two cross wires ab, cd, Plate CCXLIV. Fig. 1. with a cap e at the point of intersection, and hav ing 4 sharp points, as a, b, c, d, turned in the same direc tion, be nicely balanced upon a point at the top of the in sulated rod or stand ef, and if they are connected with an electrified conductor by the chain g h, they will immc• diately receive a rotatory motion. In the dark, a stream of light will issue from each point, and, from the veloci ty of rotation, these four streams will form a beautiful circle of light.
Exp. 7. Upon the preceding principle, an apparatus called the electrical orrery is constructed. If a globe S (Fig. 2.) representing the sun, has a projecting arm S b F, and if upon the pointed extremity of this arm is balanced another ball E, representing the earth car rying a wire M a, with a point a at one end, and at the other a ball representing the moon ; then, if a point b is fixed in the middle of the arm S F, and if the whole is balanced on an insulated stand, by loading one side of the globe S, let the apparatus be connected by a chain g h, with the prime conductor of an electri cal machine, and a rotatory motion will be given to both the arms, so that the moon will move round the earth E. w bile the earth performs its revolution round the sun S Exp. 8. Insert five arms of wire, having their extrc mitics pointed and turn( LI in the Fame die ection, into a piece of wood supported upon a point at the top of a glass as shewn in I 'lg. 3. 'l'o one of the sc amts, which is longer than the rest, suspend a glayi clapper by a silk thread, and behind it a mod c d. Let 8 bells he now pla ced upon the stand, and if a ( hail) passes the upper point a to the prime conductor, the points will round, and the glass clapper will strike against the hells during its successive revolutions.
Exp. 9. Let an inclined plane he formed of two straight parallel wires, Ali, CD, and insulated by four glass pillars A. B, C, D fixed in a stand ; and let a small
wire, m n, with two little balls at its c xtremitics, be made so as to slide clown the parallel wires by the force of gravity. On a pivot in its centre, balance a wire o Jr, with two points turned in the sante direction. When the wire in n is near the I wer end of the inclined plane, connect the parallel wire with an electrified conductor by means of a chain. The wire op will immediately turn round the pivot upon ni n, and will ascend the in clined plane.
The great utility of points as conductors or thunder rods for carrying off the electricity of the atmosphere, and the theory of their mode of action, will be folly explained in other parts of the present article. Scc SECT. XI. On the Distribution of Electricity, Art. 3. p. 278.
Sir Isaac Newton, and sonie other philosophers, had long ago imagined, that electrical and magnetical attrac tion followed the inverse ratio of the cube, or sonic high er power of the distance. ./Epinus and Cavendish, in their theories of electricity, have assumed no particular law, but merely that the action decreases as the distance increases, although the former had suspected that the law was the same as that of gravity.
Lord Stanhope had endeavoured to shew, that electric actions followed the inverse ratio of the square of the dis tance, and his reasonings were by no means conclusive, and the law was still considered as undetermined.
Our countryman Dr Robison, more than 46 years ago, made a variety of experiments on this important sub ject, and determined that the law of electric attraction and repulsion was nearly in the inverse duplicate ratio of the distance. An account of his experiments was read before a public society so early as 1769, but un fortunately he did not then regard them as of sufficient importance to be laid before the public. As we conceive that Dr Robison is entitled to a share of that honour which is now given to Coulomb, we shall lay before our readers an account of the method by which ho ascertain ed the law of electrical :action.