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 machine, 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 powerfully 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 lowering 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 tinfoil, 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 lowering the plate of glass, so as to bring the electricity of the upper coating within the grasp of the lower one, is sometimes called "condensation." The electricity on one plate or sheet was figured as squeezed together, or condensed, by the attraction of the other. A special instrument, called a condenser, 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, I', by a stick of sealing-wax or glass, G; the other, P, by a metal stem, connected with the earth. The insulated plate, P', is called the collect-
