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of a frictional machine or voltaic battery. The third conductor, then, becomes the nucleus of an electric field, and as we know that an electric field cannot be produced without the expenditure of energy, the question arises where that energy comes from. A moment's consideration will show that the energy is given by our hand in carrying the cylinder to and fro. Whilst approaching the uncharged, or weakly negatively charged, cylinder to the sphere we receive energy. There is attraction, because the negative end which is attracted is always nearer than the positive end which is repelled. After the cylinder has been discharged to earth there remains only attraction, and against this attractive force the cylinder has to be pulled away. Here our hand is called upon to impart energy to the system. We electrify the third conductor by the expenditure of energy, that is, by mechanical means. The more often we carry the cylinder to and fro, the more negative electricity do we accumulate on the third conductor, but it is evident that this process cannot go on for ever. We are only able to accumulate a definite charge on the third conductor. As this becomes charged, its tube of force also