LESSONS AV ELECTRICITY 



339 



These experiments are now made by rendering the coatings of the 

 Leyden-jar movable. Such ajar may be charged, the interior coating- 

 may be lifted out and proved uuelectric. The glass may then be re- 

 moved from the outer coating and the latter proved unelectric. Re- 

 storing the jar and coatings, on connecting the two latter, the dis- 

 charge 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 bot- 

 tom to it corresj^onding 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. 



Fig. 29. 



Fig. 30. 



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 elec- 

 tricity 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 in- 

 sulating 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. 



Sec. 22. Ignition by the Electric Spark. Various attempts had 

 been vainly made by Nollet and others to ignite inflammable sub- 

 stances by the electric spark. This was first effected by Ludolf, at 



