EXPLICATION OF ARAGO’S MAGNETIC PHENOMENA. 
155 
cated, the current of electricity \yill be from P to N; or if it be moved in the 
opposite direction, from N to P ; so that as regards the motions of the wire 
past the pole, they may be reduced to two, directly opposite to each other, one 
of which produces a current from P to N, and the other from N to P. 
115. The same holds true of the unmarked pole of the magnet, except that 
if it be substituted for the one in the figure, then, as the wires are moved in 
the direction of the arrows, the current of electricity would be from N to P, 
and as they move in the reverse direction, from P to N. 
116. Hence the current of electricity which is excited in metal when moving- 
in the neighbourhood of a magnet, depends for its direction altogether upon 
the relation of the metal to the resultant of magnetic action, or to the mag- 
netic curves, and may be expressed in a popular way thus ; Let AB (fig. 25.) 
represent a cylinder magnet, A being the marked pole, and B the unmarked 
pole ; let P N be a silver knife-blade resting across the magnet with its edge 
upward, and with its marked or notched side towards the pole A; then in what- 
ever direction or position this knife be moved edge foremost, either about the 
marked or the unmarked pole, the current of electricity produced will be from 
P to N, provided the intersected curves proceeding from A abut upon the 
notched surface of the knife, and those from B upon the unnotched side. Or if 
the knife be moved with its back foremost, the current will be from N to P in 
every possible position and direction, provided the intersected curves abut 
on the same surfaces as before. A little model is easily constructed, by using 
a cylinder of wood for a magnet, a flat piece for the blade, and a piece of 
thread connecting one end of the cylinder with the other, and passing through 
a hole in the blade, for the magnetic curves: this readily gives the result of 
any possible direction. 
117- When the wire under induction is passing by an electro-magnetic pole, 
as for instance one end of a copper helix traversed by the electric current(34.), 
the direction of the current in the approaching wire is the same with that of 
the current in the parts or sides of the spirals nearest to it, and in the receding- 
wire the reverse of that in the parts nearest to it. 
118. All these results show that the power of inducing electric currents is cir- 
cumferentially excited by a magnetic resultant or axis of power, just as circum- 
ferential magnetism is dependent upon and is exhibited by an electric current. 
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