Apparatus. [Fig. 52]. Insulating table. I T ([Exp. 64]); the extra ebonite sheet, E S (No. 27); the complete flat box, F B (No. 40, 41); the charged electrophorus cover, E C ([Exp. 68]). Arrange, as shown, I T being insulated from the earth by E S. F B should rest upon a wooden table or other large conductor.
177. Directions. (A) See how many good sparks I T will take from E C. Re-charge E C at each count, and note the relative sizes of the sparks.
(B) Discharge I T by touching it with your knuckle.
178. Condensation; Condensers. As I T easily held more sparks than it would take before ([Exp. 70]), we say that its capacity has been increased. Its potential didn't increase, because that could not get greater than the potential of E C, the charging body. To describe this state of affairs, we say that the electrification was denser than before, and that it was condensed. The capacity of I T was greatly increased by the presence of another conductor, F B, insulated from I T, but "grounded." Such a combination, 2 conductors, with a dielectric between them, is called a condenser.
A condenser can hold much more electrification at a certain potential than an equal amount of surface can hold when not properly arranged. We might call a condenser a storage battery for static electricity. The capacity of a condenser depends, among other things, upon the area of the conducting surfaces, and upon the thickness and nature of the dielectric. Among the various forms of condensers may be mentioned the Leyden jar, and the fulminating pane.
179. The Leyden Jar consists of a wide-mouthed glass jar, with tin-foil pasted upon the inside and outside to within 2 or 3 inches of the top. The inner coat or conductor is connected to a knob or ball at the top by means of a chain. To charge the jar, the outer coat is connected with the earth by holding it in the hand, or by resting it upon a table while the electrification is passed to the knob. A Leyden Battery consists of 2 or more connected jars, the object being to increase the area of the surface. The jar is discharged by touching one end of a discharger ([§ 188]) to the outer coat, and swinging its other end over to the knob, when a bright spark will pass between the knob and discharger. (See [Exp. 86].)
180. Fulminating Panes, or Franklin's Plates, are practically the same as a Leyden Jar. The tin-foil, however, is pasted upon the opposite sides of a pane of glass, a margin of about an inch being left all around. One side of the pane is charged, and takes the place of the inside coat of the jar. The other side is grounded. The pane is discharged by connecting the two sheets of foil.
181. Induction Coil Condensers consist of sheets of tin-foil separated by sheets of paraffined paper, which act as the dielectric. (See [Induction Coils].)