EXPERIMENT 70. To study potential; electro-motive force.
Apparatus. The insulating table, I T, [Fig. 38]. (For details see [Exp. 64]; the electrophorus [Exp. 68]).
143. Directions. (A) Pass a spark from the thoroughly charged E C ([Exp. 68]) to I T.
(B) Recharge E C, and see how many times I T will take good sparks from it, and note the relative sizes of the sparks.
(C) As soon as I T refuses to take more sparks from E C, touch E C to see if it is completely discharged.
(D) Touch I T.
144. Pressure; Potential; Electro-motive Force. Water runs down hill. It always tries to run from a high place to a lower one. Electrification acts very much like water in this respect. We say that water has a pressure, or a head of so many feet. In speaking of a charge, we say that it has a potential, or an electro-motive force. Water may have a high or low pressure, and a charge may have a high or low potential. The greater the pressure of water, the harder it tries to break away and get somewhere; the greater the potential of a charge, the farther it will jump to your hand.
144a. Current; Spark. Electrification will easily pass from a place of high potential to one of low potential through a conductor, and when it passes we say we have an electric current, or a current of electricity. Water has no desire to flow on a dead level, and the electric current does not care to flow between two places of equal potential. The potential of the earth and of all neutral bodies is zero; that is, they have no charge, no potential; so it is very easy for a charge to escape into the earth.