Fig. 207.—Air pressure apparatus to illustrate electrical pressure.
229. Electrical pressure is a term sometimes used for difference of potential. To better understand electrical pressure consider three round tanks (Fig. 207) containing air. A is a tank holding air at 10 lbs. pressure per square inch, above atmospheric pressure, B is open to the air and hence is at atmospheric pressure while C has a partial vacuum, with 10 lbs. less pressure than that of the atmosphere. If the valve at D or E is opened a flow of air sets up until the pressures are equalized. While if the pump at P is working a difference in pressure is easily maintained. Tank A corresponds to an insulated body charged to a high positive potential; tank B, open to the air, a body connected to the earth; while tank C represents a body having a negative potential. The earth is said to have zero potential.
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).
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.
Fig. 209.—A plate condenser.