A fine wire is then twisted several times around the bottom of the needle, and the whole apparatus is then placed on the tray, the end of the wire attached to the needle being carefully arranged so as to touch the tray, a metallic connection with the tray being essential to success. If the needle can be soldered to a metal stand, or the cork covered with tinfoil, the wire is not needed. On rubbing the paper and placing it on the tray, the electricity passes up the wire into the needle, thence into the wire cross, and escapes by the bent portions of the wires, each of which should be filed to a point. In escaping it electrifies the surrounding air, and this, according to the law that “like electricities repel each other,” has a reacting force on the wire arms. Accordingly the windmill begins to turn, and may attain a tolerable rate of speed if the tray is strongly charged.
Another amusing experiment is that known as the “electrical head of hair.” The head of a wooden doll is taken, and either provided with a real head of hair, which must be combed out straight, or a quantity of cotton is fastened to it to resemble hair.
If the head is fastened to a metal stand, and placed on the tray when the excited paper is laid upon it, the hairs become charged, and consequently repel each other, causing the whole head of hair to stand erect, each hair separate from the rest, thus presenting a most remarkable appearance. For the same reason, if a heap of small pieces of paper, feathers, etc., is laid on the tray, on placing upon it the electrified paper they will jump off in all directions, each being repelled by the others, in the same way as the gold leaves of the electroscope were repelled in the first experiment. If two pieces of pith are suspended by silk threads to a support, so as to hang close to each other, on bringing near them the electrified wax or tray they will be charged and will repel each other for some time. If when charged by the wax a heated glass rod rubbed with silk is brought near to them, they will fly to it, instead of retreating. This seems to indicate a difference between the electricities of the wax and the glass, the former of which has therefore been called negative, and the latter positive.
For giving stronger shocks than the tray is capable of, we may have recourse to the apparatus known as the Leyden jar, which may be easily charged by means of the tray and excited paper. A Leyden jar is thus easily and cheaply constructed: Take an ordinary wide-mouthed pickle bottle and a cork to fit it. Cover the outside with tinfoil, which can be fastened on with gum, and should be laid on as smoothly and as free from creases as possible. Tinfoil can be procured from any chemist. The outside being finished, the inside has to be covered also, which is a work of greater difficulty. It can best be performed by cutting another piece not quite so large as that on the outside of the bottle but of the same shape, and passing into the bottle without creasing it more than can be helped, it can be arranged inside the bottle so as to fit smoothly all round. Now a piece of brass wire is to be passed through the cork, at the end of which is a brass knob, or if simply bent round it will work, though the knob is neater. At the end of the wire which is inside the bottle a brass chain is fastened so as to touch the tinfoil inside the bottle when the cork is inserted. The tinfoil inside and outside the bottle must only reach to the bottom of the neck, leaving a space between it and the cork.
The Leyden jar is now complete, and must be thoroughly warmed before charging it. When quite hot it can be charged by bringing the knob (the jar being held by the outer coating of tinfoil) near the tray, when the excited paper is laid upon it. A spark will pass between the tray and the knob, and this must be repeated several times (say twenty for a first experiment), the jar being charged more fully the more sparks are put into it. Any one now taking the jar in one hand by the outer coating and placing a finger of the other hand near the knob will receive a shock, the severity of which depends on the number of sparks put into the jar. Several people can take the shock by joining hands, the outside one on one side holding the jar, and the outside one on the other side touching the knob. Those in the middle will not feel the shock quite so strongly as those on the outside.
Fig. 2.—Bells Chimed by a Leyden Jar.
This is an example of the “quick discharge” of a Leyden jar. It can, however, also be discharged slowly, and the following experiment makes use of this faculty. Take three small bells, which can be procured at any toy shop, and remove the clappers. Now suspend two of them by wires at opposite ends of a piece of metal or stout wire about three inches long, and suspend this wire in the center by a bent wire (or wooden, if covered with tinfoil) support, which is fixed to a thick piece of board, covered with tinfoil, to act as a base.
The tinfoil must be in communication with the supporting wire, and the height of the bells must be so adjusted that when the Leyden jar is placed between them with the third bell supported on the knob (the support of the clapper will have to be removed from the bell for this purpose), all three bells will be of equal heights and about half an inch distant from each other. (The diagram Fig. 2 will explain the arrangement.) Now suspend two small brass buttons by silk threads so as to hang between the bells when the Leyden jar is placed in the center. Charge the jar with the tray and replace it in position (of course with the bell on the top); the buttons will then begin to move backwards and forwards between the bells, and the latter will keep up a vigorous chiming until the electricity of the jar is exhausted. In this experiment it is essential that the supports be of metal, or wood covered with tinfoil, as the electricity passes from the inside of the jar to the outside while it is standing upon the tinfoil, by means of the balls, and thus causes them to vibrate.
A candle which has just been blown out, leaving the wick glowing, can easily be lighted by means of the charged Leyden jar if a piece of bent wire is held touching the outer coating and the other end on one side of the wick while the knob is approached to the other, so that the spark passes through the glowing wick. In the same way spirits of wine can be lighted, and gunpowder, guncotton, etc., exploded. To do this, it is best to have two pieces of bent wire provided with handles of glass at the middle. These wires are held by the handles, one in contact with the outer coating, and the other with the inner coating, of the charged Leyden jar. On approaching the other two ends of the wires a spark passes between them, and if a small quantity of gunpowder is placed on a table and the spark is made to pass through it by approaching the wire to either side it will be fired.