io6 



NA TURE 



[Dec. I, 1887 



round each other. A singular action this, and at first 

 sight unique. All ordinary actions and reactions between 

 two bodies take place in the line joining them : the forces 

 acting between a current and a pole act exactly at right 

 angles to the line joining them. 



Helmholtz long ago (in 1847) showed that the conserva- 

 tion of energy could only be true if forces between bodies 

 varied in some way with distance and acted in the line 

 joining them. Now here is a case where the forces are 

 not in the line joining the bodies, and accordingly the 

 conservation of energy is defied : the two things will 

 revolve round each other for ever. This affords and has 

 afforded a fine field for the perpetual motionist ; and if 

 only the current would rnaintain itself without sustaining 

 power, a perpetual motion would in fact be attained. 

 But this after all is scarcely remarkable, for the same 

 may be said of a sewing-machine or any other piece of 

 mechanism : if only it would continue to go without 

 sustaining power it would be a perpetual motion. Attend 

 to pole and current only, and the energy is not conserved, 

 it is perpetually being wasted ; but include the battery as 

 an essential part of the complete system, and the mystery 

 disappears : everything is perfectly regular. 



Fig. 17. — A long flexible conductor twisting itself into a spiral round a 

 powerful bar-magnet. 



The easiest way perhaps of showing the rotation of a 

 conductor conveying a current round a magnetic pole is 

 to take an 8-feet-long piece of gold thread, such as mili- 

 tary officers stitch upon their garments, and hanging it 

 vertically supply it with as strong a current as it will 

 stand. Then bring near it a vertical bar-magnet, and 

 instantly you will see the thread coil itself into a spiral, 

 half of it twisting round the north end of the bar, and half 

 twisting the other way round the south end (Fig. 17). 



If the magnet were flexible and the conductor rigid, the 

 magnet would in like manner coil itself in a spiral round 

 the current : the force is strictly mutual. A rigid magnet 

 put near a stiff conductor shows only the last remnants of 

 this action : it sets itself at right angles to the wire, and 

 approaches its middle to touch it, but that is all it 

 can do. 



The experiment with the flexible gold thread is simple, 

 satisfactory, and striking, but the rotatory properties con- 

 nected with a magnet may be illustrated in numbers of 

 other ways. Thus, pivot a disk at its centre, and arrange 

 some light contact to touch its edge, either at one point 



or all round, it matters not ; then supply a current to disk 

 from centre to circumference, and bringing a bar-magnet 

 near it along its axis, or, better, two bar-magnets, with 



Fig. 18. — Pivoted disk with radial current, revolving in a magnetic field 

 and winding up a weight. The current is supplied to the axle by screw 

 A, and leaves the rim by mercury trough M. The same apparatus 

 obviously serves to demonstrate currents induced by motion ; both 

 directly and by the damping effect. 



opposite poles one on each side, near the contact place 

 of the rim, the disk at once begins to rotate (Figs. 18 

 and 19). 



Fig. 19. — Another pivoted disk with flange to dip into liquid so as to make 

 contact all round its rim. It rotates when a magnet is brought above or 

 below ; or even in the field of the earth. 



Instead of a disk one may use a single radius of it, viz. 

 a pivoted arm (Fig. 20) dipping into a circular trough of 

 mercury ; or we may use a light sphere rolling on two 



Fig. 20. — A couple of radii of the above disTc provided with points to dip 

 into mercury, and rotating constantly under the influence of the sttel 

 magnet a. 



concentric circular lines of railway (Gore's arrangement. 

 Fig. 21). In every case rotation laegins as soon as a 

 magnet is brought near. 



