Feb. 1 6, 1888] 



NA TURE 



3^7 



The easiest way to picture the effect of iron is to think 

 of its wheels as some two or three iiundred times as mas- 

 sive as those of air, so that their energy and momentum 

 are very great. 



That which is commonly called magnetic permeability 

 may in fact be thought of as a kind of inertia, an inertia 

 per unit volume ; though how it comes to pass that the 

 ether inside iron is endowed with so great inertia one 

 cannot say. Perhaps it is that the iron atoms themselves 

 revolve with the electricity, perhaps it is something quite 

 different. Whatever the peculiar behaviour of iron, 

 nickel, &c., be due to, it must be something profoundly 

 interesting and important as soon as our knowledge of 

 their molecular structure enables us to perceive its 

 nature. 



Induction in Conductors not originally carrying Currents 

 but moving in a Magnetic Field. 



To explain the currents induced in a conductor moving 

 through a uniform magnetic field is not quite easy, be- 

 cause none of the diagrams lend themselves naturally 

 and simply to the idea of circuits changing in form or size. 



If we take a rigid circuit in a magnetic field, like Fig. 

 45, and revolve it out of its plane 180'', it is obvious that a 

 current will be excited in it, for the process is essentially 

 the same as if the conductor were kept still and the field 

 reversed. 



Fig. 45. — Section of a uniform magnetic field with two rails and a slider 

 in it. If the slider be moved to or fro, the wheels inside get initially, 

 compressed or extended, and thereby gain or lose energy respectively 

 thus exciting the state of sUp known as induced current. 



But to understand the current excited in a closed 

 circuit when a portion of it moves across the lines so as to 

 embrace a greater number of them, one has to take into 

 account the fact that the inside whirls are expanding and 

 doing work in forcing the conductor away, while the outer 

 whirls are resisting the motion, and being thereby com- 

 pressed and rendered more energetic. Thus the wheels 

 inside revolve slightly slower as the circuit expands, and 

 those outside revolve slightly quicker. Both these processes 

 cause a slipping of the gearing, first all round the inside 

 and then all through the substance of the wire, whereby 

 positive electricity moves forward in one direction round 

 the circuit, the negative moving oppositely ; and so a 

 current is accounted for. It is not to be supposed, how- 

 ever, that any finite expansion of the wheels really 

 occurs : the motion is rapidly equalized by diffusion 

 through the wire, and fresh wheels come in round it from 

 outside ; hence directly after the conductor has stopped 

 moving the field is again steady, but with many more 

 wheels inside the contour than it possessed at first. 



Representation of an Electrostatic Field again, and 

 superposition of it on a perpendicular Magnetic Field. 



An electrostatic strain is, we know, caused by a dis- 

 placement of positive electricity one way along the lines 



of force, and by an equal displacement of negative the 

 other way. Half the process was indicated crudely in 

 Fig. 6 ; we may now represent it rather more fully with 

 the help of our elastic cells by Fig. 46. 



Fig. 46— a portion of an electrostatic iield between two oppositely charged 

 bodies, wilh its lines of force going from right to left, and showing a 

 tension along and a pressure at right angles to them, due to the elasti- 

 city of the cells (which elasticity may be due to their containing fluid in 

 a slate of whirl). Magnetic lines of force perpendicular to the paper are 

 also shown in section. While this magnetic field was being excited and 

 propagated from below upwards, a slight strain would be produced in 

 the elastic cells, like but immensely less than that shown ; as contrasted 

 with its normal condition (Fig. 37). Conversely, while this electrostatic 

 strain was being produced, the positive whirls would be infinitesimally 

 quickened and the negative ones retarded during the displacement, thus 

 producing a minute magnetic effect. If the medium is not magnetized, 

 the whirls aie not necessarily absent, only faced all w:iys. 



Here the positive cells have been pulled one way, the 

 negative the other way ; and when the distorting force is 

 removed, the medium tends to spring back to its normal 

 condition, exerting an obvious tension on bodies attached 

 to it in the direction of its hnes of force, its elongated 

 direction, and an obvious pressure in all perpendicular 

 directions, its compressed directions. 



Now, if all the cells are full of parallel whirls, as in the 

 preceding magnetic diagrams, it is not improbable that 

 this electrostatic distortion or " shear " of the medium 

 may affect its magnetic properties slightly, and that, if the 

 direction of electrostatic strain were rapidly reversed, 

 a small magnetic oscillation would also ensue ; but the 

 exact details of these mutual actions are difficult to 

 specify at present. 



Disruptive Discharge. 



Disruptive discharge may be thought of as a pulling of 

 the shaded cells violently along past the others ; the 

 process being accompanied by a true disruption — a sort 

 of electrolysis — of the medium, and a passage of the 

 two electricities in opposite directions along the line of 

 discharge. 



Consider the locomotion of any one horizontal row of 

 shaded cells in Fig. 46 during the occurrence of such a 

 disruption of the medium. The cells slide on towards 

 the right, and, as they slide, the spin of the negative cells 

 above them is retarded while that of those below them 

 is accelerated ; consequently a true magnetic effect is 

 produced, just like that accompanying a current, and a 

 disruptive discharge has therefore all the magnetic 

 properties of a current. 



Effects of a Moving Charge. 



This locomotion of a set of positive cells, or of negative 

 cells the other way, as just considered, is very near akin 

 to the motion of a charge through a dielectric medium. 



A charge can only exist at the boundary between a 

 dielectric and a conductor, or at least between one di- 

 electric and another of greater density. So, when a 

 charged body moves along with extreme rapidity, it can 

 be thought of as exciting a rotation in the cells rnost 

 closely in contact with it greater than that which it excites 

 in the opposite kind of cells, and thus produces the whirl 

 proper to a magnetic field. Thus does a moving charge 

 behave just like a current of a certain strength. 



It may be, indeed, that this is the customary way of 

 exciting a voltaic current ; for the chemical forces in a 

 cell cause a locomotion of charged atoms, and thus set 



