EVOLUTION OF ELECTRICITY FROM MAGNETISM. 
135 
helix, for then a second action occurs. When the magnet is introduced, the 
needle at the galvanometer is deflected in a certain direction; but being in, 
whether it be pushed quite through or withdrawn, the needle is deflected in a 
direction the reverse of that previously produced. When the magnet is passed 
in and through at one continuous motion, the needle moves one way, is then 
suddenly stopped, and finally moves the other way. 
41. If such a hollow helix as that described (34.) be laid east and west (or in 
any other constant position), and a magnet be retained east and west, its marked 
pole always being one way; then whichever end of the helix the magnet goes 
in at, and consequently whichever pole of the magnet enters first, still the 
needle is deflected the same way: on the other hand, whichever direction is 
followed in withdrawing the magnet, the deflection is constant, but contrary 
to that due to its entrance. 
42. These effects are simple consequences of the law hereafter to be de¬ 
scribed (114). 
43. When the eight elementary helices were made one long helix, the effect 
was not so great as in the arrangement described. When only one of the eight 
helices was used, the effect was also much diminished. All care was taken to 
guard against any direct action of the inducing magnet upon the galvano¬ 
meter, and it was found that by moving the magnet in the same direction, and 
to the same degree on the outside of the helix, no effect on the needle was 
produced. 
44. The Royal Society are in possession of a large compound magnet formerly 
belonging to Dr. Go win Knight, which, by permission of the President and 
Council, I was allowed to use in the prosecution of these experiments : it is at 
present in the charge of Mr. Christie, at his house at Woolwich, where, by 
Mr. Christie’s kindness, I was at liberty to work ; and I have to acknowledge 
my obligations to him for his assistance in all the experiments and observations 
made with it. This magnet is composed of about 450 bar magnets, each fifteen 
inches long, one inch wide, and half an inch thick, arranged in a box so as to 
present at one of its extremities two external poles (fig. 5.). These poles pro¬ 
jected horizontally six inches from the box, were each twelve inches high and 
three inches wide. They were nine inches apart; and when a soft iron cylinder, 
three quarters of an inch in diameter and twelve inches long, was put across 
