368 



NA TURE 



[Feb. 1 6, 1888 



up a field, which, spreading out in the way Prof. Poynting 

 has sketched, reaches every part of the metallic circuit 

 and excites the current there. 



Electrostatic Ejects of a Moving or Varying Magnetic 

 Field. 



Just as we have seen that a moving or varying electro- 

 static field may produce slight magnetic effects, so one 

 can perceive that a moving or varying magnetic field 

 brings about something of the nature of an electrostatic 

 strain. 



For a spreading out field is continually propagating 

 the rotation on from one layer of wheels to the next. If 

 there is any slip, we thus get induced currents, and the 

 rate of propagation is comparatively slow, being a kind 

 of diffusion ; but even if there is not any slip, yet, unless 

 the wheel-work is absolutely rigid, the rate of propaga- 

 tion will not be infinite. The actual rate of propagation 

 is very great, which shows that the rigidity of the wheels 

 is very high in proportion to their inertia, but it is not 

 infinite ; and accordingly the propagation of rotation is 

 accompanied by a temporary strain. One part of the 

 field is in full spin, another more distant part is as yet 

 unreached by the spin ; between the two we have the 

 region of strain, the wheel-work being distorted a little 

 while taking up the motion. Thus does a spreading out 

 magnetic field cause a slight and temporary electrostatic 

 strain, at right angles both to the direction of the lines of 

 force and to the direction of their advance. 



Generation of a Magnetic Field. Induction in Closed 

 Circuits. 



Picture to oneself an unmagnetized piece of iron : its 

 whirls are all existent, but they are shut up into little 

 closed circuits, and so produce no external effect. Mag- 

 netize it slightly, and some of the closed circuits open out 

 and expand, with one portion of them in the air. Mag- 

 netize it strongly, and we have a whole set of them opened 

 out into vortex cores, still with the whirl round them, and 

 constituting the common magnetic lines of force. There is 

 no need to think of iron and steel in this connection. In air 

 or any substance the whirls are still present, though much 

 fewer or feebler, and their axes ordinarily form little closed 

 circuits — it may be inside the atoms themselves. But 

 wrap a current-conveying wire round them, and at once 

 they open out into the lines of force propei: to a circular 

 current. 



Again, think of an iron ring, or a hank of wire as bought 

 at an ironmonger's : wrap a copper wire several times 



Fig. 47. — Closed magnetic circuit like Fig. 42, with a single-ring second.iry 

 circuit, and another open secondary loop; alsa with a short conducting- 

 rod standing up in it. 



round it, as a segment of a Gramme ring is wound (Fig. 

 47), and pass a current. The closed vortices in the iron 

 at once expand : a portion of each flashes out and 

 across the air-space inclosed by the ring (not by any 

 means confining itself to a plane, of course), and enters 

 the ring on the opposite side ; so that directly the current 

 is steady the lines all lie inside the iron again, but now 

 inclosing an area — the area of the ring — instead of being 

 shut up into infinitesimal links.- In a sense the iron is 

 still unmagnetized, for its lines of force still form closed 

 contours within it, and none protrude any part of them- 

 selves into the air, except for irregularities. But in 

 another sense it is highly and permanently magnetized 



round and round in itself, the magnetism being not easy 

 to get out of it again, except by judiciously arranged 

 reverse currents. 



It is now like one great electric vortex rmg instead of 

 like a confused jumble of microscopic ones. Its section 

 was shown in Fig. 42. 



During the variable period, while the current is increas- 

 ing in strength, or while it is being reversed, the region 

 inclosed by the ring and all around it is full of myriads of 

 expanding lines of force flashing across, broadside on, 

 from one side of the iron to the other, and there 

 stopping. It is the presence of these moving lines, 

 changing rapidly from a "simply-connected" into a 

 " multiply-connected " state, or vice versa, which causes 

 the powerful induced currents of " secondary generators." 



In every case of varying magnetic field, in fact, we 

 have lines moving broadside on, propagating their whirl, 

 and more or less disturbing the medium through which 

 they move. 



Next consider a moving or spinning magnet. Its lines 

 travel with it, and, being closed curves, they also must 

 move broadside through the field, so that in this case we 

 may expect Just the same effect as can be obtained from 

 a varying magnetic field. 



If a broadside-moving line of force cut across a con- 

 ductor, its motion is delayed, for its wheels slip and only 

 gradually get up a whirl inside the ill-geared substance ; 

 thus, as we know, causing an induced current (see Fig. 43). 



If a conducting ring is looped with the iron ring 

 previously mentioned, as a snap-hook is looped with an 

 eye, then every expanding vortex, while the ring is being 

 magnetized, has necessarily to cut through the conducting 

 ring once and no more, no matter what its shape or size. 

 The electromotive force of induction is in this case there- 

 fore perfectly definite, and simply proportional to the 

 number of turns made by the secondary round the core 

 of the ring (Fig. 47). 



Instead of supposing a closed conducting secondary 

 circuit, imagine an open one : there is still an E.M.F. in 

 it, though rather less than before because a few of the 

 expanding lines flash through the gap and produce no 

 effect, so the electricity must surge to and fro in the 

 conductor as water surges up and down in a tilted 

 trough, and a small condenser attached to the free ends 

 will be alternately charged and discharged. The gap 

 might become so large that nothing is left but a short rod 

 (Fig, 47) : in this also similar oscillations would occur. 



But now suppose no secondary conductor at all ; 

 nothing but dielectric inclosed by the ring. In it there 

 must be an electric displacement excited every time the 

 magnetism of the ring is reversed. It may be an oscil- 

 latory displacement, but still on the whole in one direction 

 during rise of magnetism, and in an opposite direction 

 during reversal of magnetism. A charged body delicately 

 suspended within the ring may feel the effect of the minute 

 electrostatic strain so magnetically produced. 



To see the mode in which an electrostatic displace- 

 ment arises in the space embraced by the ring we have 

 only to turn to Fig. 42, and look at the set of wheels along 

 the line A B separating one half the section from the other. 

 They cannot steadily rotate either way, for they are urged 

 in opposite directions by the two halves ; in other words, 

 there is no magnetic field near such a ring, as is well 

 known ; but, nevertheless, during a change of magnetism, 

 while the whirls inside are changing in speed, the rub on 

 the dielectric necessary for checking the outer wheels of 

 the conductor is either increased or diminished ; and if 

 the wheels have any elastic "give" in them, as we know 

 they have, the electrostatic strain in the field is thereby 

 altered during the varying stage of the magnetism. 



Oliver J. Lodge. 



END OF PART III. 



[To be continued.) 



