Explanation of the Cause of Magnetic and Electric Rotations
In 1824 Arago published his experiments on the magnetism manifested by various substances during rotation. I was much struck with the announcement, and immediately set up some apparatus in my own workshop in order to witness the facts thus announced.
My friend Herschel, who assisted at some of the earliest experiments, joined with me in repeating and varying those of Arago. The results were given in a joint paper on that subject, published in the Transactions of the Royal Society in 1825.
I had previously made some magnetic experiments on a large magnet which would, under peculiar management, sustain about 32½lbs. It was necessary to commence with a weight of about 28lbs., and then to add at successive intervals additional weights, but each less and less than the former.
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 that the original cause was sufficient for the production of all the observed effects.
These experiments and their explanation were printed in the Phil. Trans. 1826. But they met with so little acceptance in England that I had ceased to contend for them against more popular doctrines, and was too deeply occupied with other inquiries to enter on their defence. Several years after, during a visit to Berlin, taking a morning walk with Mitscherlich, I asked what explanation he adopted of the magnetic rotations of Arago. He instantly replied, “There can be no doubt that yours is the true one.”
It will be a curious circumstance in the history of science, if an erroneous explanation of new and singular experiments in one department should have led to the prevision of another similar set of facts in a different department, and even to the explanation of new facts at first apparently contradicting it.