LESSONS IN ELECTRICITY. 



307 



1 will now ask you to charge your 

 Dutch metal electroscope (fig. 7) posi- 

 tively by rubbed gutta-percha, and to 

 charge it negatively 7 by rubbed glass. A 

 moment's reflection will enable you to do 

 it. You bring your excited bodv near : 

 the same electricity as that of the excited 

 body is driven over the leaves, and they 

 diverge by repulsion. Touch the elec- 

 troscope, the leaves collapse. Withdraw 

 your finger, and withdraw afterwards the 

 excited body : the leaves then diverge 

 with the opposite electricity. 



The simplest way of testing the quality 

 of electricity is to charge the electroscope 

 with electricity of a known kind. If, on 

 the approach of the body to be tested, 

 the leaves diverge still wider, Ilio lvr<* 

 and the body are similarly electrified. 

 The reason is obvious. 



Omitting the last experiment, the 

 wealth of knowledge which these re- 

 searches involve might be placed within 

 any intelligent boy's reach by the wise 

 expenditure of half-a-crovvn. 



Once firmly possessed of the principle 

 of induction and versed in its application, 

 the difficulties of our subject will melt 

 away before us. In fact oar subsequent 

 work will consist mainly in unravelling 

 phenomena by the aid of this principle. 



Without a knowledge of this principle 

 wo could render no account of the attrac- 

 tion of neutral bodies by our excited 

 tubes. In reality the attracted bodies 

 arc not neutral : they are first electrified 

 by influence, and it is because they are 

 thus electrified that they are attracted. 



This is the place to refer more fully 

 to a point already alluded to. Neutral 

 bodies, as just stated, arc attracted, be- 

 cause they arc really converted into elec- 

 trified bodies by induction. Suppose a 

 body to be feebly electrified positively, 

 and that you bring your rubbed glass 

 tubo t:> bear upon the body. You clear- 

 ly sec that the induced negative electricity 

 may be strong enough to mask and over- 

 come the weak positive charge possessed 

 by the body. We should thus have two 

 bodies electrified alike, attracting each 

 other. This is the danger against which 

 I promised to warn you in 10, where 

 the test of attraction was rejected. 



Wo will now apply the principle of in- 

 duction to explain a very beautiful inven- 

 tion, made known by the celebrated 



Voltain 1775. 



15. The Mectrophorus. 



Cut a circle, T, fig. 23, 6 inches in 

 diameter out of sheet zinc, or out of 

 common tin. Heat it at its centre by the 



FIG. 23. 



flame of a spirit-lamp or of a candle. 

 Attach to it there a stick of sealing-wax, 

 H, which, when the metal cools, i* to 

 serve as an insulating handle. You have 

 now the lid of the electrophorus. A 

 resinous surface, or what is simpler a 

 sheet of vulcanized india-rubber, p, or 

 even of hot brown paper, will answer for 

 the plate of the electrophorus. 



Rub your " plate" with flannel, or 

 whisk it briskly with a fox's brush. It 

 is thereby negatively electrified. Place 

 thy " lid " of your olectrophorus on the 

 excited surface : it touches it at a few 

 points only. For the most pait lid and 

 plate arc separated by a film of air. 



The excited surface acts by induction 

 across this film upon the lid, attracting 

 its positive and repelling its negative 

 electricity. You Lnvo in fact in the lid 

 two layers of electricity, the lower one, 

 which is " bound," positive ; tho upper 

 one, which is " free," negative. ^Lift 

 the lid : the electricities iiow again to- 

 gether ; neutrality is restored, and your 

 lid fails to attract your balanced lath. 



Once more plnce the lid upon the ex- 

 cited surface : touch it with the finger. 

 What occurs ? You ought to know. 

 Tho free electricity, , which is negative, 

 will escape through your body to tho 

 earth, leaving the chained positive be- 

 hind. 



Now lift the lid by the handle : what 

 is its condition ? Again I say you ought 



