54 ELEMENTARY LESSONS ON [CHAP. i. 



attract one another, though neither they themselves nor 

 the electricities on them can pass through the glass. If 

 a pith-ball charged with electricity be hung inside a 

 dry glass bottle, and a rubbed glass rod be held outside, 

 the pith-ball will rush to the side of the bottle nearest to 

 the glass rod, being attracted by the + electricity thus 

 brought near it. If a pane of glass be taken, and a piece 

 of tinfoil be struck upon the middle of each face of the 

 pane, and one piece of tinfoil be charged with + , and 

 the other with electricity, the two charges will attract 

 one another across the glass, and will no longer be found 

 to be free. If the pane is set up on edge, so that neither 

 piece of tinfoil touches the table, it will be found that 

 hardly any electricity can be got by merely touching either 

 of the foils, for the charges are " bound," so to speak, 

 by each other's attractions ; each charge is inducing the 

 other. In fact it will be found that a great deal more 

 of the two electricities may be put into these two pieces 

 of tinfoil than could possibly be put into either of them 

 if it were stuck to a piece of glass alone, and then elec- 

 trified. In other words, the capacity of a conductor is 

 I/ greatly increased when it is placed near to a conductor 

 charged with the opposite kind of electricity. If its 

 capacity is increased, a greater quantity of electricity 

 may be put into it before it is charged to a high degree 

 of potential. Hence, such an arrangement for holding 

 a large quantity of electricity may be called an accu- 

 mulator of electricity. 



48. Condensers. Nextly, suppose we have two 

 brass discs, A and B (Fig. 30), set upon insulating 

 stems, and that a glass plate is placed between them. 

 Let B be connected by a wire to the knob of an electrical 

 machine, and let A be joined by a wire to "earth." The 

 + electricity on B will act inductively across the glass 

 plate on A, and will repel + electricity into the earth, 

 and attract electricity on to the nearest face of A. 

 This electricity on A will attract the + electricity of 



