Page:Popular Science Monthly Volume 9.djvu/354

 332 the Leyden philosophers correctly stated the conditions necessary to the success of the experiment. Hence the phial received the name of the Leyden-phial, or Leyden-jar.

The discovery of Kleist and Cunæus excited the most profound interest, and the subject was explored in all directions. Wilson, in 1746, filled a phial partially with water, and plunged it into water, so as to bring the water surfaces, within and without, to the same level. On charging such a phial the strength of the shock was found greater than had been observed before.

Two years subsequently Dr. Watson and Dr. Bevis noticed how the charge grew stronger as the area of the conductor in contact with the outer surface increased. They substituted shot for water inside the jar, and obtained substantially the same effect. Dr. Bevis then coated a plate of glass on both sides with silver-foil, within about an inch of the edge, and obtained from it discharges as strong as those obtained from a phial containing half a pint of water. Finally, Dr. Watson coated his phial inside and out with silver-foil. By these steps the Leyden-jar reached the form which it possesses to-day.

It is easy to repeat the experiment of Dr. Bevis. Procure a glass plate nine inches square; cover it on both sides with tin-foil six inches square. Connect one side with the earth and the other with the machine. Charge and discharge: you obtain a brilliant spark.

In our experiment with the golden fish (Fig. 22), we employed a common form of the Leyden-jar, only with the difference that to get to a sufficient distance from the glass, so as to avoid the attraction of the fish by the jar itself, the knob was placed higher than usual. But, with a good flint-glass tumbler, a piece of tin-foil, and a bit of wire, you can make a jar for yourself. Bad glass, remember, is not rare. In Fig. 24 you have such a jar. T is the outer, T' the inner coating, reaching to within an inch of the edge of the tumbler G. W is the bit of wire fastened below by wax, and surmounted by a knob, which may be of metal, or of wax or wood, coated with tin-foil. In charging the jar you connect the outer coating with the earth—say with a gas-pipe or a water-pipe—and present the knob to the conductor of your machine. A few turns will charge the jar. It is discharged by laying one knob of a "discharger" against the outer coating, and causing the other knob to approach the knob of the jar. Before contact, the electricity flies from knob to knob in the form of a spark.

A "discharger" suited to our means and purposes is shown in Fig. 25. H is a stick of sealing-wax: W W a stout wire bent as in the figure, and ending in the knobs B B'. These may be of wax coated with tin-foil. Any other light conducting knobs would of course answer. The insulating handle H protects you effectually from the shock.

18. Explanation of the Leyden-Jar.—The principles of electrical induction with which you are now so familiar will enable