LENZ ON ELECTRO-MAGNETISM. 609 



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* 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 im- 

 mediate 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 

 cj^linder, 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 momen- 

 tancous 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 ofl" 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 diflierence 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 



* [n this memoir the measures are always expressed in Englisli inclies, ex- 

 cejit wlicn olliciwise rcmarke<l. 



