LESSONS IN ELECTRICITY. 335 



Carry a fine wire from the upper sheet of tin-foil to your electroscope. 

 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 necessary, until you see a slight divergence of the Dutch metal 

 leaves. Or, connecting the weight with the conductor of your ma- 

 chine, turn very carefully until the slight divergence is observed. 

 What is the condition of things here ? You have poured, say, positive 

 electricity on to the upper sheet of metal. It will act inductively 

 across the glass upon the under sheet, the positive fluid of which will 

 escape to the earth, leaving the negative behind. You see before 

 your mind's eye two 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 expect to occur? 

 Freed from the grasp of the lower layer, the electricity of the upper 

 one will diffuse itself over the electroscope so promptly and power- 

 fully that, if you are not careful, you will destroy the instrument by 

 the mutual repulsion of its leaves. 



Practise this experiment, which is perfectly easy and perfectly 

 beautiful, by loweVing and lifting the glass plate, and observing the 

 corresponding rhythmic action of the leaves of the electroscope. 

 The experiment was shown here twelve years ago to boys and girls 

 who are now men and women. 



Common tin-plate may be used in this experiment, instead of tin- 

 foil, and a sheet of vulcanized India-rubber instead of the pane of 

 glass. Or, simpler still, for the tin-foil a sheet of common unwarmed 

 foolscap may be employed. Satisfy yourself of this. Spread a sheet 

 of foolscap on a table ; lay the plate of glass upon it, and spread a 

 leaf of foolscap, less than it in size, on the plate of glass. Connect 

 the leaf with the electroscope, and charge it exactly as you charged 

 the tin-foil. On lifting the 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 covered 

 by an insulating varnish, you will obtain with it the results obtained 

 with the tin-foil, tin, and foolscap. 



The withdrawal of the electricity from the electroscope, by lower- 

 ing the plate of glass, so as to bring the electricity of the upper coat- 

 ing within the grasp of the lower one, is sometimes called " condensa- 

 tion." The electricity on one plate or sheet was figured as sqtieezed 

 together, or condensed, by the attraction of the other. A special 

 instrument, called a condense?', is constructed by instrument-makers 

 to illustrate the action here explained. 



You may readily make a condenser for yourself. Take two circles, 

 P P\ Fig. 27, of tin or of sheet-zinc, and support the one, V', by a 

 stick of sealing-wax or glass, G ; the other, P, by a metal stem, con- 

 nected with the earth. The insulated plate, P\ is called the collect- 



