VOLTA-ELECTRIC AND MAGNETO-ELECTRIC INDUCTION. 
187 
within it. These curves, although different in form, are perfectly analogous to 
those existing between two contrary magnetic poles opposed to each other ; 
and when a second wire, parallel to that which carries the current, is made to 
approach the latter (18.), it passes through magnetic curves exactly of the same 
kind as those it would intersect when carried between opposite magnetic poles 
(109.), in one direction ; and as it recedes from the inducing wire, it cuts the 
curves around it in the same manner that it would do those between the same 
poles if moved in the other direction. 
233. If the wire NP (fig. 40.) have an electric current passed through it in 
the direction from P to N, then the dotted ring may represent a magnetic curve 
round it, and it is in such a direction that if small magnetic needles be placed 
as tangents to it, they will become arranged as in the figure, n and s indicating- 
north and south ends (44. note.). 
234. But if the current of electricity were made to cease for a while, and 
magnetic poles were used instead to give direction to the needles, and make 
them take the same position as when under the influence of the current, then 
they must be arranged as at fig. 41 ; the marked and unmarked poles a b 
above the wire, being in opposite directions to those a' 6' below. In such 
a position therefore the magnetic curves between the poles a b and a! b' have 
the same general direction with the corresponding parts of the ring magnetic 
curve surrounding the wire N P carrying an electric current. 
235. If the second wire pn (fig. 40.), be now brought towards the principal 
wire, carrying a current, it will cut an infinity of magnetic curves, similar in 
direction to that figured, and consequently similar in direction to those between 
the poles ab of the magnets (fig. 41.), and it will intersect these current curves 
in the same manner as it would the magnet curves, if it passed from above 
between the poles downwards. Now, such an intersection would, with the 
magnets, induce an electric current in the wire from p to n (114.) ; and there- 
fore as the curves are alike in arrangement, the same effect ought to result 
from the intersection of the magnetic curves dependent on the current in the 
wire N P; and such is the case, for on approximation the induced current is in 
the opposite direction to the principal current (19.). 
236. If the wire p n! be carried up from below, it will pass in the opposite 
direction between the magnetic poles; but then also the magnetic poles them- 
2 b 2 
