his discovery was that after the battery had thus run down he could by passing a current from some external source through the battery in the opposite direction restore the plates to their former state so that the battery was ready once more to give out a current.
So long as the current necessary thus to restore his battery could only be derived from some other battery this invention had little value or promise. But the Dynamo had come and by means of it any form of motive power could be made to generate the necessary return current. The Planté battery therefore became a reservoir of electric power which could be filled by the use of motive power derived from any convenient source just as a reservoir of water can be filled from a brook or stream. Now it is necessary that a submarine when submerged in the open sea should be kept moving in order to maintain its longitudinal stability as well as to escape pursuit. The motive force necessary for this must be obtained from some source which requires no air and gives off no