The principle upon which the machine works will be best understood by reference to [Fig. 5]. In this diagram the inner circle represents the front plate, with neutralizing brushes A and B, and the outer one represents the back plate, with brushes C and D. The sectors are shown heavily shaded. E and F are the pairs of combs, and the plates rotate in the direction of the arrows. Let us suppose one of the sectors at the top of the back plate to have a slight positive charge. As the plates rotate this sector will come opposite to a front plate sector touched by brush A, and it will induce a slight negative charge on the latter sector, at the same time repelling a positive charge along the rod to the sector touched by brush B. The two sectors carrying the induced charges now move on until opposite back plate sectors touched by brushes C and D, and these back sectors will receive by induction positive and negative charges respectively. This process continues as the plates rotate, and finally all the sectors moving towards comb E will be positively charged, while those approaching comb F will be negatively charged. The combs collect these charges, and the discharging rods K and L become highly electrified, K positively and L negatively, and if they are near enough together sparks will pass between them.
Fig. 5.—Diagram to illustrate working of a Wimshurst Machine.
At the commencement we supposed one of the sectors to have a positive charge, but it is not necessary to charge a sector specially, for the machine is self-starting. Why this is the case is not yet thoroughly understood, but probably the explanation is that at any particular moment no two places in the atmosphere are in exactly the same electro-static condition, so that an uneven state of charge exists permanently amongst the sectors.
The Wimshurst machine provides us with a plentiful supply of electricity, and the question naturally arises, “Can this electricity be stored up in any way?” In 1745, long before the days of influence machines, a certain Bishop of Pomerania, Von Kleist by name, got the idea that if he could persuade a charge of electricity to go into a glass bottle he would be able to capture it, because glass was a non-conductor. So he partly filled a bottle with water, led a wire down into the water, and while holding the bottle in one hand connected the wire to a primitive form of electric machine. When he thought he had got enough electricity he tried to remove his bottle in order to examine the contents, and in so doing he received a shock which scared him considerably. He had succeeded in storing electricity in his bottle. Shortly afterwards the bishop’s experiment was repeated by Professor Muschenbrock of Leyden, and by his pupil Cuneus, the former being so startled by the shock that he wrote, “I would not take a second shock for the kingdom of France.” But in spite of shocks the end was achieved; it was proved that electricity could be collected and stored up, and the bottle became known as the Leyden jar. The original idea was soon improved upon, water being replaced by a coating of tinfoil, and it was found that better results were obtained by coating the outside of the bottle as well as the inside.
As now used the Leyden jar consists of a glass jar covered inside and outside with tinfoil up to about two-thirds of its height. A wooden lid is fitted, through which passes a brass rod terminating above in a brass knob, and below in a piece of brass chain long enough to touch the foil lining. A Leyden jar is charged by holding it in one hand with its knob presented to the discharging ball of a Wimshurst machine, and even if the machine is small and feeble the jar will accumulate electricity until it is very highly charged. It may now be put down on the table, and if it is clean and quite dry it will hold its charge for some time. If the outer and inner coatings of the jar are connected by means of a piece of metal, the electricity will be discharged in the form of a bright spark. A Leyden jar is usually discharged by means of discharging tongs, consisting of a jointed brass rod with brass terminal knobs and glass handles. One knob is placed in contact with the outer coating of foil, and the other brought near to the knob of the jar, which of course is connected with the inner coating.
The electrical capacity of even a small Leyden jar is surprisingly great, and this is due to the mutual attraction between opposite kinds of electricity. If we stick a piece of tinfoil on the centre of each face of a pane of glass, and charge one positively and the other negatively, the two charges attract each other through the glass; and in fact they hold on to each other so strongly that we can get very little electricity by touching either piece of foil. This mutual attraction enables us to charge the two pieces of foil much more strongly than if they were each on a separate pane, and this is the secret of the working of the Leyden jar. If the knob of the jar is held to the positive ball of a Wimshurst, the inside coating receives a positive charge, which acts inductively on the outside coating, attracting a negative charge to the inner face of the latter, and repelling a positive charge to its outer face, and thence away through the hand. The electricity is entirely confined to the sides of the jar, the interior having no charge whatever.
Leyden jars are very often fitted to a Wimshurst machine as shown at A, A, [Fig. 4], and arranged so that they can be connected or disconnected to the collecting combs as desired. When the jars are disconnected the machine gives a rapid succession of thin sparks, but when the jars are connected to the combs they accumulate a number of charges before the discharge takes place, with the result that the sparks are thicker, but occur at less frequent intervals.
It will have been noticed that the rod of a Leyden jar and the discharging rods of a Wimshurst machine are made to terminate not in points, but in rounded knobs or balls. The reason of this is that electricity rapidly leaks away from points. If we electrify a conductor shaped like a cone with a sharp point, the density of the electricity is greatest at that point, and when it becomes sufficiently great the particles of air near the point become electrified and repelled. Other particles take their place, and are electrified and repelled in the same way, and so a constant loss of electricity takes place. This may be shown in an interesting way by fastening with wax a needle to the knob of a Wimshurst. If a lighted taper is held to the point of the needle while the machine is in action, the flame is blown aside by the streams of repelled air, which form a sort of electric wind.