These tiny sparks from the electrophorus, or the bigger discharges of an electrical machine, can be stored in a simple apparatus called a Leyden jar, which was discovered by accident.  One day Cuneus, a pupil of Muschenbroeck, professor in the University of Leyden, was trying to charge some water in a glass bottle by connecting it with a chain to the sparkling knob of an electrical machine.  Holding the bottle in one hand, he undid the chain with the other, and received a violent shock which cast the bottle on the floor.  Muschenbroeck, eager to verify the phenomenon, repeated the experiment, with a still more lively and convincing result.  His. nerves were shaken for two days, and he afterwards protested that he would not suffer another shock for the whole kingdom of France.

The Leyden jar is illustrated in figure 9, and consists in general of a glass bottle partly coated inside and out with tinfoil F, and having a brass knob K connecting with its internal coat.  When the charged plate or conductor of the electrophorus touches the knob the inner foil takes a positive charge, which induces a negative charge in the outer foil through the glass.  The corresponding positive charge induced at the same time escapes through the hand to the ground or “earth.”  The inner coating is now positively and the outer coating negatively electrified, and these two opposite charges bind or hold each other by mutual attraction.  The bottle will therefore continue charged for a long time; in short, until it is purposely discharged or the two electricities combine by leakage over the surface of the glass.

To discharge the jar we need only connect the two foils by a conductor, and thus allow the separated charges to combine.  This should be done by joining the outer to the inner coat with a stout wire, or, better still, the discharging tongs T, as shown in the figure.  Otherwise, if the tongs are first applied to the inner coat, the operator will receive the charge through his arms and chest in the manner of Cuneus and Muschenbroeck.

Leyden jars can be connected together in “batteries,” so as to give very powerful effects.  One method is to join the inner coat of one to the outer coat of the next.  This is known as connecting in “series,” and gives a very long spark.  Another method is to join the inner coat of one to the inner coat of the next, and similarly all the outer coats together.  This is called connecting “in parallel,” or quantity, and gives a big, but not a long spark.

Of late years the principle of induction, which is the secret of the Leyden jar and electrophorus, has been applied in constructing “influence” machines for generating electricity.  Perhaps the most effective of these is the Wimshurst, which we illustrate in figure 10, where pp are two circular glass plates which rotate in opposite directions on turning the handle.  On the outer rim of each is cemented a row of radial slips of metal at equal