Page:Popular Science Monthly Volume 9.djvu/361

 Rh These experiments are now made by rendering the coatings of the Leyden-jar movable. Such a jar may be charged, the interior coating-may be lifted out and proved unelectric. The glass may then be removed from the outer coating and the latter proved unelectric. Restoring the jar and coatings, on connecting the two latter, the discharge passes in a brilliant spark.

Make a jar with movable coatings thus: Roll cartridge-paper round a good flint-glass tumbler, G, Fig. 29, to within about an inch of the top. Paste down the edge of the paper, and put a paper bottom to it corresponding to the bottom of the glass. Coat the paper, T, inside and out with tin-foil. Make a similar coating, T', for the inside of the tumbler, attaching to it an upright wire, W, ending in a hook. You have then, to all intents and purposes, a Leyden-jar.

Charge the jar, and by means of a rod of glass, sealing-wax, or gutta-percha, lift out the interior coating. It will carry a little electricity away with it. Place it upon a table and discharge it wholly. Lift the glass by the hand out of the outer coating. Neither of the coatings now shows the slightest symptom of electricity. Restore the tumbler to its outer coating, and, by means of the hook and insulating rod, restore the inner coating to its place. Discharge the jar: you obtain a brilliant spark. The electricity which produces this spark must have been resident in and on the glass.

You can charge your jar with a rubbed glass rod, though a machine, in good working order will do it more rapidly.

22. Ignition by the Electric Spark.—Various attempts had been vainly made by Nollet and others to ignite inflammable substances by the electric spark. This was first effected by Ludolf, at