518 



LESSONS IN ELECTRICITY. 



coating of a ifiird, E F ; what occurs ? 

 The third jar will obviously be charged 

 with the electricity repelled from the 

 outer coating 1 of tho second. Of course 

 we need not stop here. We may have a 

 long series of insulated jars, the outer 

 coating of each being connected with the 

 inner coating of the next succeeding one. 

 Connect the outer coating of the last jar 

 i K by a wire c with the caith, and charge 

 the first jar. You charge thereby the 

 entire series of jars. In this simple way 

 you master practically, and grasp tho 

 theory of Franklin's celebrated " cascade 

 battery." 



You must see that before making this 

 important experiment you could really 

 have predicted what would occur. This 

 power of prension is one of the most 

 striking characteristics of science. 



22. Novel Ley den Jars of the Simplest 

 J^orm. 



Possessed of its principles, we can re- 

 duce the Lcydon jar to far simpler forms 

 than any hitherto dealt with. Spread a 

 sheet of tin-foil smoothly upon a table, 

 and Jay upon the foil a* pane of glass. 

 Remember that the glass, as usual, must 

 be dry. Stick on to the glass by seal- 

 ing-wax two loops of narrow silk ribbon, 

 by which the pane may be lifted ; and 

 then lay smoothly upon the glass n sec- 

 ond sheet of tin-foil, less than the pane 

 in size, leaving a rim of uncotercd glass 

 all round. Carry a fine wire from the 

 upper sheet cf tin-foil to your electro- 

 scope. A little weight will keep the end 

 of the wire attached to the tin-foil. 



Rub this weight with your excited 

 glass tube, two or three times if neces- 

 sary, until you see a slight divergence of 

 the Dutch metal leaves. Or connecting 

 the weight with the conductor of your 

 machine, turn very carefully until the 

 slight divergence is observed. What is 

 the condition of things here ? You have 

 poured, say positirc electiicity on to the 

 upper sheet of metal. It acts "inducti vo ly 

 across the glass upon the under sheet, the 

 positive fluid of which escapes to the 

 earth, leaving the negative behind. You 

 see before your mind's eye twx> layers 

 holding each other in bondage. Now 

 take hold of your loops and lift the glass 

 plate, so as to separate the upper tin-foil 

 from the lower. What would you ex- 



pect to occur ? Freed from the grasp of 

 the lower layer, the electricity of the up- 

 per one will diffuse itself over the elec- 

 troscope so promptly and powerfully, 

 that if you are not careful you will de- 

 stroy the instrument by the mutual repul- 

 sion of its leaves. 



Practise this experiment, which is a 

 very old one of mine, by lowering and 

 lifting the glass plate, and observing the 

 corresponding rhythmic action of the 

 leaves of the electroscope. 



Common tin-plate may be used in this 

 experiment instead of tin-foil, and a 

 sheet of vulcanized india-rubber instead 

 of the pano of glass. Or simpler still, 

 for the tin-foil a sheet of common un- 

 warmed foolscap may be employed. 

 Satisfy yourself of this. Spread a sheet 

 of foolscap on a table ; lay the plate of 

 glass upon it, an I spread a leaf of fools- 

 cap, less than the glass in size, on the 

 plate of glass. Connect tho loaf with 

 the electroscope, and charge it, exactly 

 as you charged the tin-foil. On lifting 

 tho glass with its leaf of foolscap, the 

 leaves of the electroscope instantly fly 

 apart ; on lowering the glass they again 

 fall together. Abandon the under sheet 

 altogether, and make the table the outer 

 coating ; if it be not of very dry wood, 

 or corered by an insulating varnish, you 

 will obtain with it the results obtained 

 with the tin-foil, tin, and foolscap. 

 Thus by the simplest means we illustrate 

 great principles. 



The withdrawal of the electricity from 

 the electroscope, by lowering the plate of 

 glass, so as to bring the electricity of the 

 upper coating within the grasp of the 



FIG. 41. 



