134 MR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 
posed to be the end of the wire going to the positive pole of the battery, or 
that end towards which the zinc plates face, and N the negative wire. Such a 
current would have converted the cylinder into a magnet of the opposite kind 
to that formed by contact with the poles A and 13 ; and such a current moves 
in the opposite direction to the currents which in M. Ampere’s beautiful theory 
are considered as constituting a magnet in the position figured *. 
39. But as it might be supposed that in all the preceding experiments of 
this section it was by some peculiar effect taking place during the formation of 
the magnet, and not by its mere virtual approximation, that the momentary in¬ 
duced current was excited, the following experiment was made. All the similar 
ends of the compound hollow helix (34.) were bound together by copper wire, 
forming two general terminations, and these were connected with the galva¬ 
nometer. The soft iron cylinder (34.) was removed, and a cylindrical magnet, 
three quarters of an inch in diameter and eight inches and a half in length, 
used instead. One end of this magnet was introduced into the axis of the helix 
(fig. 4.), and then, the galvanometer-needle being stationary, the magnet was 
suddenly thrust in; immediately the needle was deflected in the same direc¬ 
tion as if the magnet had been formed by either of the two preceding processes 
(34. 36.). Being left in, the needle resumed its first position, and then the 
magnet being withdrawn the needle was deflected in the opposite direction. 
These effects were not great; but by introducing and withdrawing the magnet, 
so that the impulse each time should be added to those previously communi¬ 
cated to the needle, the latter could be made to vibrate through an arc of 
180° or more. 
40. In this experiment the magnet must not be passed entirely through the 
* The relative position of an electric current and a magnet is by most persons found very difficult to 
remember, and three or four helps to the memoty have been devised by M. Ampere and others. I ven¬ 
ture to suggest the following as a very simple and effectual assistance in these and similar latitudes. 
Let the experimenter think he is looking down upon a dipping needle, or upon the pole of the earth, and 
then let him think upon the direction of the motion of the hands of a watch, or of a screw moving direct; 
currents in that direction round a needle would make it into such a magnet as the dipping needle, or 
would themselves constitute an electro-magnet of similar qualities; or if brought near a magnet would 
tend to make it take that direction; or would themselves be moved into that position by a magnet so 
placed; or in M. Ampere’s theory are considered as moving in that direction in the magnet. These 
two points of the position of the dipping-needle and the motion of the watch-hands being remembered, 
any other relation of the current and magnet can be at once deduced from it. 
