6. If a positively charged cloud floats over a tree which is a good conductor of electricity will the tree be charged? Show diagram. Explain. (4) POTENTIAL, CAPACITY AND THE ELECTRIC CONDENSER =227. Conditions Causing a Movement of Electricity.=--In the study of conductors and insulators it was observed that an electric charge moved along the conducting rod to the electroscope. This _movement_ of _electricity_ along a conductor is a result of great practical importance. We will now consider the conditions that produce the "flow" or "current" of electricity. Let two electroscopes stand near each other. Charge one, _C´_ (Fig. 206), strongly and charge the other slightly. If now a light stiff wire attached to a stick of sealing wax be placed so as to connect the tops of the electroscopes, the leaves of _C_ will partly close while those of _D_ will open slightly, thus indicating a movement of electricity from _C_ to _D_ along the wire. The movement was from a place of greater degree of electrification to one of less. [Illustration: FIG. 206.--Electricity flows from high to low potential.] =228. Potential.=--The _potential_ of an electrified body is its _degree_ of _electrification_. Therefore, it is said that electroscope _C_ mentioned above has a greater potential than electroscope _D_. The movement of electricity is from a place of greater or _high_ potential to one of lesser or _low_ potential. If two bodies are at the _same_ potential there will be found no movement of electricity between them. A _difference_ of _potential_ between two points connected by a conductor is therefore the _necessary condition_ for an electric current. Just as heat is transmitted along a conductor from a place of high to one of lower temperature, so electricity is transmitted along a conductor from a place of high to one of low potential. Thus potential in electricity corresponds to temperature in heat. One is the "degree of electrification," the other, "the degree of hotness." [Illustration: 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_.