You partly reproach us here with not sufficiently esteeming Ampère’s experiments on electromagnetism. Allow me to extenuate your opinion a little on this point. With regard to the experiments, I hope and trust that due weight is allowed to them; but these you know are few, and theory makes up the great part of what M. Ampère has published, and theory in a great many points unsupported by experiments when they ought to have been adduced. At the same time, M. Ampère’s experiments are excellent, and his theory ingenious; and, for myself, I had thought very little about it before your letter came, simply because, being naturally sceptical on philosophical theories, I thought there was a great want of experimental evidence. Since then, however, I have engaged on the subject, and have a paper in our “Institution Journal,” which will appear in a week or two, and that will, as it contains experiment, be immediately applied by M. Ampère in support of his theory, much more decidedly than it is by myself. I intend to enclose a copy of it to you with the other, and only want the means of sending it. I find all the usual attractions and repulsions of the magnetic needle by the conjunctive wire are deceptions, the motions being not attractions or repulsions, nor the result of any attractive or repulsive forces, but the result of a force in the wire, which instead of bringing the pole of the needle nearer to, or further from the wire, endeavours to make it move round it in a never ending circle and motion whilst the battery remains in action. I have succeeded not only in showing the existence of this motion theoretically, but experimentally, and have been able to make the wire revolve round a magnetic pole, or a magnetic pole round the wire, at pleasure. The law of revolution, and to which all the other motions of the needle and wire are reducible, is simple and beautiful. Conceive a portion of connecting wire north and south, the north end being attached to the positive pole of a battery, the south to the negative. A north magnetic pole would then pass round it continually in the apparent direction of the sun, from east to west above, from west to east below. Reverse the connections with the battery, and the motion of the pole is reversed; or if the south pole be made to revolve, the motions will be in the opposite directions, as with the north pole.