ELECTRO-MAGNETISM. 



11 



deflected towards the'wire, the force that 

 tends to bring it back to the position of 

 rest increases till it reaches its maximum 

 at the position SU, where the needle 

 points directly to the wire. Carrying it 

 still further to the left, the rotatory force 

 again diminishes, till it arrives at the 

 position BS, perpendicular to BW, 

 where, being directed to the exact centre 

 of motion, it is reduced to nothing. This 

 position of the needle, therefore, is also 

 one of equilibrium ; but it differs from 

 the former in being an unstable equi- 

 librium ; for if the needle be dis- 

 turbed ever so little from its position 

 on either side, it will acquire a ten- 

 dency to proceed onwards in that direc- 

 tion, and will move away from the point 

 B. At A, for instance, the rotatory 

 force acts upon it by the lever S a, urging 

 it towards U, and causing it ultimately 

 to settle at N. At C, again, it is urged 

 towards D by a similar force, propor- 

 tional to S c, and which, increasing as 

 the needle advances, carries it to V, and 

 finally brings it round to N. 



(33.) It may here be remarked, that 

 the rotations of the needle are in oppo- 

 site directions in these two portions of 

 the circumference ; for, in the remote 

 part, BVN, the motion is similar to 

 that of the hands of a watch ; in the 

 nearer part, BUN, it is in the contrary 

 direction. The lines \VB and "WN, 

 drawn from W to the points where the 

 needle is in equilibrium, being at right 

 angles to the respective radii BS and 

 NS, are tangents to the circle at B and 

 N, and the circumference is divided by 

 these points into two unequal portions, 

 so that the needle, in passing from B to 

 N, by the operation of the tangential 

 force emanating from \V, as indicated 

 by the arrows in the figure, has to tra- 

 verse a longer distance when moving in 

 the remote than in the nearer part of 

 the circle. The disproportion between 

 these two arcs continually increases as 

 the wire is brought nearer to the circle. 

 When very near, as shown in fig. 14, 



Fig. 14. 



the arc BUN is very small, compared 

 with BVN ; yet, if the needle be placed 

 ever so little on the other side of B, it 

 will immediately recede from that point, 

 as if repelled by the wire, and will pro- 

 ceed to describe the larger portion of the 

 circle, in order to arrive at N, a position 

 which it might have reached by a much 

 more direct course had it described the 

 arch BUN. 



(34.) The singular preference thus 

 shown by the needle for a very circuitous 

 path, in reaching its destination, when 

 it appeared free to take the shorter line 

 that leads to it, appeared exceedingly pa- 

 radoxical to those who first observed it, 

 and excited much astonishment. But 

 the explanation we have given shews 

 clearly that it is nothing more than the 

 direct result of the peculiar law of 

 electro- magnetic force, which is charac- 

 terized by the tangential direction of its 

 agency. 



(35.) If the wire be supposed to pass 

 through the circumference itself, as in 

 fig. 15, that portion of the circumfer- 



Fig. 15. 



v\ 



ence BN, which was comprehended be- 

 tween the two tangents, and in which 

 the needle was urged to turn in a direc- 

 tion contrary to that of its revolution in 

 the rest of the circle, is now reduced to 

 a mere point ; and the needle, when 

 placed ever so little to the left of that 

 point, will move round the entire circle, 

 and even when it arrives at this point, 

 can hardly be said to settle there, for 

 the slightest movement in the same di- 

 rection will again place it under the 

 influence of the same impulse, which 

 will, therefore, carry it round a second 

 time. The very momentum it has ac- 

 quired in this motion will be sufficient 

 to transport it beyond this neutral point, 

 and to maintain it in a state of perpetual 

 revolution. Should the wire be actually 

 within the circle, as mfig. 16, then the 

 rotatory force will remain constantly in 

 the same^direction in every part of the 

 circle, and, according to theory, the 



