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44 takes place we do not know, but as a matter of convenience we assume that there is only one current, namely, that which flows from the positive to the negative conductor, much in the same way as water will always flow from the higher to the lower level.

Electricity, being an imponderable entity (in reality merely a form of energy), cannot be connected mentally with any conception of level, such as is legitimate in the study of the movement of heavy bodies. Nevertheless it is convenient to introduce a somewhat analogous conception to "high" and "low" when dealing with electrical problems, and this conception is that of "electric potential." Just as water tends to flow from the higher level to the lower level, so positive electricity has the tendency to flow from the conductor of higher to that of lower potential. The mechanical meaning of the term potential will be discussed in the following chapter; for the present it must suffice to note that as long as the two conductors are kept at a difference of potential by the working of the frictional machine, a current of electricity will flow from the positive to the negative conductor through the wire joining them.

The current obtainable from such a machine