The following was the apparatus I employed for my experiments. A multiplier (with a very sensible double needle of Nobili) of seventy-four coils of copper wire of 0·025 of an English inch in thickness[1] was placed in connection by means of conducting wires with the electro-motive spirals, so that the horseshoe magnet which acted on the spirals was at a distance of nineteen feet from the multiplier, and had no immediate influence on its needles. I had assured myself of this by previous experiments. The horseshoe magnet consisted of five single bent steel bars, firmly connected with one another by screws; the middle one protruded at the ends about 0·7 of an inch; it might together with the armature weigh somewhat more than twenty-two pounds. The length of the bars was twenty-three inches, the breadth 0·8, and the thickness 0·22; the middle one projecting beyond the others was 0·4 in thickness; the distance of the arms was 1·64 inch. In order to be able to approach and remove the spirals, and at the same time to read off the deviation of the needle without any aid, I constructed my apparatus in the following way:—I did not cover the multiplier with its bell glass, but with a glass cylinder open at both ends, and closed these by means of a plate of mirror glass; I then placed over it a good mirror under an inclination of 45°, and from a point near the magnet I observed by means of a good Munich telescope the reflected image of the scale of the multiplier. The reading off was thus performed very precisely, and was more certain than with the naked eye close to the scale, because at this distance and with a fixed position of the eye the parallel axis of the index which stands at some distance from the graduated circle may be considered as evanescent. The method of exciting the electric current M'as the same as that given by Nobili: I wound the electromotive wire about a soft iron cylinder, which served as an armature and was filed smooth at those places where it was laid on the magnet, and laid it then on the magnet, or removed it suddenly from it, by which the magnetism arising at the moment, or vanishing again in the armature, thus produced the momentancous electric current. But as the removal of the armature can be performed in a more certain, prompt, and uniform manner than the placing of it on, I have in all my following experiments only given the results which were caused by the taking off of the armature, or the sudden removal of the magnetism in the iron. I must here at the same time remark that in my experiments it made no difference whether the magnetism of the iron disappeared really and entirely all at once, or there still remained a part, provided only the remaining quantity of magnetism was the same after each removal. I frequently convinced
- ↑ In this memoir the measures are always expressed in English inches, except when otherwise remarked.
