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 distanee 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 scaie, 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 was 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 momen- 
taneous electric current. But as the removal of the armature can be 
performed ina 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, ex- 
cept when otherwise remarked. 
