320 



(An account of Franklin's discoveries 

 was given by him in a series of letters 

 addressed to "Peter Collmson, Esq.* 

 F.R.S., from 1747 to 1754). 



So much for history ; but yon arc to 

 verify the history by repeating Franklin's 

 experiments. Place water in a wide 

 glass vessel ; place ;i second glass vessel 

 within the first, and till it to the same 

 height with water. Connect the outer 

 water by a wire with the earth, and the 

 inner water by a wire with the electric 

 machine. One or two turns furnish a suf- 

 ficient charge. Removing the inner wire, 

 and dipping one finger into the outside 

 and the- other into the. inside water, a 

 smart shock is felt. This was Franklin's 

 first experiment. 



Pass on to the second. Coat a, glass 

 jar uiih tin-foil (not too high) ; fill it to 

 the same height with water, and place it 

 on india-rubber cloth. Charge it by 

 connecting the outside coating with the 

 earth, and the water inside (by means of 

 a stem cemented to the bottom of the vir 

 and ending above in a knob) with an 

 electric machine. You obtain a bright 

 spark on discharging. This proves your 

 apparatus to be in good order. 



Recharge. Take hold of the charged 

 jar with the india-rubber, and pour the 

 water into a second similar jar. No sen- 

 sible charge is imparted to the latter. 

 Pour fresh unelectrified water into the 

 first jar, and discharge it. The retention 

 of the charge is shown by a brilliant spaik. 

 Be careful in these experiments, or yon 

 will fail, as I did at first. The edge of 

 the jar out of which the water is poured 

 has to be surrounded by a band of bibu- 

 lous paper to catch the final drop, which, 

 trickling down, would discharge the jar. 



Experiments like those of Franklin are 

 now made by rendering the coatings of 

 the Leydcn jar movable. Such a jar be- 

 ing 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 con- 

 necting the two latter, the discharge 

 passes in a brilliant spaik. 



Make a jar with movable coatings 

 thus : Roll cartridge paper round a good 

 flint-glass tumbler, c, fig. 43, to within 

 about an inch of the top. Paste down 

 the lower edge of the paper, and put a 



LE33O:73 IN ELECTRICITY. 



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 



PIG. 43. 



w, ending in a hook. You have then to 

 all intents and purposes a Leyden jar. 



Put the pieces together and charge the 

 jar. By means of a rod of glass, seal- 

 ing-wax, or gutta-percha, lift out the in- 

 terior coating. It will carry .a little elec- 

 tricity away with it. Place it upon a 

 tnble and discharge it wholly. Then by 

 the hand lift the glass out of the outer 

 coating. Neither of the coatings now 

 shows the slightest symptom of electric- 

 ity. 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. 



Here, as in all other cases, you can 

 charge your jar with a rubbed glass tube, 

 though a machine in good working order 

 will do it more rapidly. With *' Cot- 

 trell's rubber," described in the next 

 section, you may greatly exalt the per- 

 formance of your glass tube. 



24. Ignition by the Electric Spark. 

 CottrelVs Rubber. The Tube-ma- 

 chine. 



Various attempts had been vainly made 

 by Nollet and others to ignite inflam- 



