This led me to an explanation of the cause of those rotations, which I still venture to think is the true cause, although it is not so recognized by English philosophers.
The history is a curious one, and whether the cause which I assigned is right or wrong, the train of thought by which I was led to it is valuable as an illustration of the mode in which the human mind works in its progress towards new discoveries.
The first experiment, showing that the weight suspended might be increased at successive intervals of time, was stated in most treatises on magnetism. But the visible fact impressed strongly on my mind the conclusion that the production and discharge of magnetism is not instantaneous, but requires time for its complete action. It appeared, therefore, to me that this principle was sufficient for the explanation of the rotations observed by Arago.
In the following year it occurred to me that electricity possessed the same property, namely, that of requiring time for its communication. I then instituted a new series of experiments, and succeeded, as I had anticipated, in producing electric rotations. But a new fact now presented itself: in certain cases the electric needle moved back in the contrary direction to that indicated by the influences to which it was subjected. Whenever this occurred the retrograde motion was always very slow. After eliminating successively by experiment every cause which I could imagine, the fact which remained was, that in certain cases there occurred a motion in the direction opposite to that which was expected. But whenever such a motion occurred it was always very slow. Upon further reflection, I conjectured that it might arise from the screen, interposed between the electric and the needle itself, becoming electrified possibly in the opposite direction. New experiments confirmed this view and proved
