EXPLICATION OF ARAGO’S MAGNETIC PHENOMENA. 
149 
ends of the needle when affected merely by terrestrial magnetism ; the end 
N is therefore the marked pole (44.). The whole instrument was protected 
by a glass jar, and stood, as to position and distance relative to the large 
magnet, under the same circumstances as before (45.). 
88. All these arrangements being made, the copper disc was adjusted as in 
fig. 7? the small magnetic poles being about half an inch apart, and the edge 
of the plate inserted about half their width between them. One of the galva- 
nometer wires was passed twice or thrice loosely round the brass axis of the 
plate, and the other attached to a conductor (86.), which itself was retained 
by the hand in contact with the amalgamated edge of the disc at the part im- 
mediately between the magnetic poles. Under these circumstances all was 
quiescent, and the galvanometer exhibited no effect. But the instant the 
plate moved, the galvanometer was influenced, and by revolving the plate 
quickly the needle could be deflected 90° or more. 
89. It was difficult under the circumstances to make the contact between 
the conductor and the edge of the revolving disc uniformly good and exten- 
sive ; it was also difficult in the first experiments to obtain a regular velocity 
of rotation : both these causes tended to retain the needle in a continual state 
of vibration ; but no difficulty existed in ascertaining to which side it was de- 
flected, or generally, about what line it vibrated. Afterwards, when the expe- 
riments were made more carefully, a permanent deflection of the needle of 
nearly 45° could be sustained. 
90. Here therefore was demonstrated the production of a permanent cur- 
rent of electricity by ordinary magnets (57.). 
91. When the motion of the disc was reversed, every other circumstance re- 
maining the same, the galvanometer needle was deflected with equal power as 
before ; but the deflection was on the opposite side, and the current of electri- 
city evolved, therefore, the reverse of the former. 
92. When the conductor was placed on the edge of the disc a little to 
the right or left, as in the dotted positions fig. 9, the current of electricity was 
still evolved, and in the same direction as at first (88. 91.). This occurred to 
a considerable distance, i. e. 50° or 60° on each side of the place of the mag- 
netic poles. The current gathered by the conductor and conveyed to the gal- 
vanometer was of the same kind on both sides of the place of greatest inten- 
