\tion, we should have , or double the previous action. We might even construct wheel-work in such a manner that the velocities of the systems should be in the ratio of , and that the poles should meet times, during one revolution. The action would then be , and this increase would be gained by purely geometrical means. This is a simple deduction from the fact that velocity does not enter into magnetic attraction. I have not as yet availed myself of this advantage in the construction of magnetic apparatus, since there are some remarkable circumstances, as we shall see hereafter, not sufficiently cleared up, and which may give rise to considerable modifications.
6.
The inversion of the poles is an object of the greatest importance. This inversion should take place instantaneously, and precisely at the place where the poles are situated opposite to one another. The mechanism intended to produce this operation should be put in motion by the apparatus itself, but no element should be introduced which is dependent in a geometrical manner upon the rotatory movement of the system. The velocity of the motion, however great it may be, should not at all affect this operation. The well-known bascule, an ingenious invention of M. Ampère, which is so advantageously employed in electro-magnetic experiments, cannot be employed in the magnetic apparatus with a continuous circular motion; for the number of inversions, in a given time, cannot be considerable without requiring extraordinary means; and even these means will not guarantee the certain result of an advancing and receding movement, repeated as frequently as may be necessary. I shall not recount here all the attempts I have made, both numerous and expensive, to arrive at the important result of an inversion of the poles, exact and precise, divested at once of every element depending on the velocity. But it is necessary to say that the greatest difficulties arose by employing mercury, as is usual in electro-magnetic experiments to form and to break metallic contact. In the liquid state the adhesion of the mercury to the metallic body plunged into it and afterwards withdrawn, varies with the rapidity of the motion and with the purity of the mercury. Frequently—I may say always—the inversion takes place too soon or too late, and thus gives rise to an attraction or repulsion, in a contrary direction to the rotation. Moreover it is very difficult to preserve the mercury pure when in contact with other metals; and even the purest mercury is disposed to oxidize easily under the influence of the electric sparks. These sparks are produced, under favourable circumstances, on establishing metallic contact, and always on breaking it. The result is, that the surface of the
