If a number of lengths of brass wire, supplied with balls at the extremities, are supported on glass legs and arranged in a line, with a little pith ball attached to a thread hanging from each end of the length of brass wire, the effect of induction is shown very nicely; and when an excited glass rod is brought towards one end of the series, the rising of the pith balls to each other betrays the change which has occurred in the electrical state of the brass wires by the mere neighbourhood of the excited glass tube. The glass tube is electrified positively, and attracts the negative electricity from the brass wire towards the end nearest to it; the other extremity of the brass wire is found to be in the positive state, and this re-acting on the next, and so on throughout the lengths, completes the electrical disturbance in the whole series. (Fig. 172.)
Fig. 172.
The lengths of brass wire supported on glass rod pillars indented by blowpipe, so as to retain the brass wires with the pith balls hanging from each series, the letters p and n mean Positive and Negative, and the signs for these terms are placed above. The letters p and n are painted on the blocks which support the glass rods.
Thirty-second Experiment.
If an insulated brass rod (such as has been described in the last experiment) is touched by the finger whilst under induction, it remains permanently electrified on the removal of the disturbing electrified body; and it is on this principle that the useful electrical machine called the Electrophorus is constructed. This constant electrical machine—for it will remain in action during weeks and months if kept sufficiently dry—was invented by Volta in the year 1774, and has been brought to great perfection by Mr. Lewis M. Stuart, of the City of London School; so that with a little additional apparatus the whole of the fundamental principles of electricity can be demonstrated. It consists of a flat brass or tin circular dish about two feet in diameter and half an inch deep, which is filled with a composition of equal parts of black rosin, shell-lac, and Venice turpentine; the rosin and the Venice turpentine being first melted together, and the shell-lac added afterwards, care of course being taken that the materials do not boil over and catch fire, in which case the pot must be removed from the heat, and a piece of wet baize or other woollen material thrown over it. Another tin or brass circular plate of twelve inches diameter, and supported in the centre with a varnished glass handle nine inches long, is also provided, and the resinous plate being first excited by several smart blows with a warm roll of flannel, the plate held by the glass handle is now laid upon the centre of the resinous one, and if removed directly afterwards, does not afford the electric spark; but if, whilst standing upon the excited resinous plate, it is touched, and then removed by the glass handle, a powerful electric spark is obtained; and this may be repeated over and over again with the like results, provided the plate with the glass handle is touched with the finger just before lifting it from the resinous plate. (Fig. 173.) The electricity excited on the resinous plate is not lost, and by induction sets up the opposite condition in the plate with the glass handle. The resinous plate, being excited with negative electricity, disturbs the electrical quiescence of the upper plate, and positive electricity is found on the surface touching the resinous plate, and negative electricity on the upper one, so that when it is removed without being touched, the two electricities come together again, and no spark is obtained; but if, as already described, the upper plate is touched whilst under induction, then positive electricity appears to pass from the finger to the negative electricity on the upper side of the plate, when the two temporarily neutralize each other, and then, when the plate is removed, the excess of electricity derived from the earth through the finger becomes apparent. Induction requires no sensible thickness in the conductors, and can be just as well produced on a leaf of gold as on the thickest plate of metal; and it should be remembered that non-conductors do not retain their state of electrical excitation when the disturbing cause is removed, whereas conductors possess this power, and this fact brings us to the consideration of the Leyden jar.
Fig. 173.
a a. Large circular tin or brass disc with turned-up edge half an inch deep, and containing the resinous mixture b, which is rubbed with the warm flannel. c c. The upper plate supported by the glass handle d, a pith ball attached to a wire shows the electrical excitation, and the spark is supposed to be passing to the hand e.