Now just as compressed air will be pushed into the atmosphere (as from _A_ to _B_) while air at atmospheric pressure will if possible be forced itself into a partial vacuum (as from _B_ to _C_), so electricity at a positive potential will tend to move to a place at zero potential, while that at zero potential tends to move to a place of negative potential. Bodies at the _same_ potential as the earth, or at zero potential, are also said to be _neutral_. Those positively electrified have a positive potential, those negatively electrified have a negative potential. As in gases, movement always tends from higher pressure (potential) to lower pressure (potential). [Illustration: FIG. 208.--The metal plate gives the electroscope a greater surface and hence greater capacity.] =230. Capacity.=--If we have a 100-gallon tank and a 10-gallon tank connected by a pipe both filled with compressed air, the larger tank will contain ten times as much air as the smaller at the _same pressure_ since it has ten times the capacity, or, if the two tanks are separated and the same amount of air is contained in each, the pressure of the air contained in the small tank will be ten times that in the large one. The _electrical capacity_ of a conductor is in some respects similar to the capacity of a tank for air. Since, however, electrical charges are upon the surface of a body, its capacity depends in part upon the extent of surface. For example, if a charge is taken from a charged rubber rod by a proof plane to an electroscope a certain divergence of the leaves will be noticed. If a circular metal plate several times the diameter of the top of the electroscope is laid upon the latter (see Fig. 208), and a charge equal to that used before is brought to the electroscope, the leaves show less divergence than before, showing that the _same charge gives a lower potential when placed upon a body of greater capacity_. [Illustration: FIG. 209.--A plate condenser.] [Illustration: FIG. 210.--A condenser of several plates.] =231. The electric condenser= is a device having a large electrical capacity consisting of parallel conductors separated by good insulators. It has been devised to enable one to obtain a large electrical charge upon a body of convenient size. Such an apparatus is of great practical value in many experiments and operations. Its construction involves the principle of electrostatic induction in which a charge of one kind attracts and "holds" strongly a charge of opposite kind near it. In its simplest form it consists of two parallel conductors separated from each other (Fig. 209). The upper plate has been charged negatively. This has given the lower plate a positive charge by induction, since the latter is connected to the earth. These positive and negative charges hold or "bind" each other so that a large quantity may be accumulated. To increase the capacity of a condenser, several plates are used connected as in Fig. 210.