chap, i.] CONDENSERS. 1 1 



sealing wax, and the discharging rods n m connected 

 with the conductors, are brought into contact. This 

 armature induces positive electricity on the surface of 

 the glass plate next to it, and negative electricity on 

 the surface opposite the brass points. The elec- 

 tricity of the glass causes the brass points to discharge 

 + electricity on to the glass, and so to become nega- 

 tively charged. The glass plate is now turned, and, 

 when the part positively charged from the brass 

 points comes opposite the second armature, it charges 

 it positively, and induces the opposite brass points to 

 discharge electricity and to remain positively 

 charged. The portion of the plate, now negatively 

 charged, still being revolved, returns to the first 

 armature, increases its - - charge, and so heightens its 

 inductive action. The knobs n in in connection with 

 the brass points, are thus charged, the one with -+- 

 the other with electricity. On the knobs being 

 separated, and the plate rapidly turned, a series of 

 sparks dart across from one knob to the other. 



Condensers are instruments for concentrating 

 a large quantity of electricity on a small surface, an 

 action also effected by induction. The Leyden jar is 

 the best example. It consists of a 

 glass jar or bottle, coated inside 

 and outside with tinfoil, up to a 

 few inches from the neck. The 

 mouth is stopped with a cork or 

 a plug of hard wood, in which is 

 fixed a metal rod, terminating out- 

 side in a knob, and having a 

 chain hanging from it inside Fig. 6.-Leyden Jar. 

 and touching the inner li.iing 

 of tinfoil. The jar is charged by connecting the 

 outer coat with the ground, as, also, by holding it so 

 that the hand touches that coating, and presenting the 

 knob to the conductor of a friction machine. If the 



