534 



NATURE 



{Oct. 6, 1887 



belongs not to the water in the jelly but to the mode in 

 which it is entangled in its meshes. However all this is 

 a great and difficult question into which we shall be able 

 to enter with more satisfaction twenty years hence. 



Provisionally we will accept as a working hypothesis 

 the idea of the ether consisting of electricity in a state 

 of entanglement similar to that of water in jelly ; and we 

 are driven to this view by the exigencies of mode i, the 

 electrostatic or strain method of examining the properties 

 of electricity, because otherwise the properties of insula- 

 tors are hard to conceive. If it turn out that space is a 

 conductor, which seems to me highly improbable, then 

 we must fall back upon the other view that it is rigid only 

 for infinitesimal vibrations, and iluid for steady forces. 



Return now to the consideration of electrostatics. 

 We are to regard ourselves as living immersed in an 

 infinite all-permeating ocean of perfect incompressible 

 fluid (or liquid), as fish live in the sea ; but this is not all, 

 for if that were our actual state we should have no more 

 notion of the existence of the liquid than deep-sea fish 

 have of the medium they swim in. If matter were all 

 perfectly conducting, it would be our state : in a perfectly 

 free ocean there is no insulation — no obstruction to flow 

 of liquid : it is the fact of insulation that renders electro- 

 statics possible. We could obstruct the flow and store 

 up definite quantities of a fluid in which we were totally 

 submerged by the use of closed vessels of course. But 

 how could we pump liquid from one into another 

 so as to charge one positively and another negatively ? 

 Only by having the walls elastic : by the use of elastic 

 Ijags, and elastic partitions across pipes. And so we can 

 represent a continuous insulating medium (like the atmo- 

 sphere or space) by the analogy of a jelly, through which 

 liquid can only flow by reason of cracks and channels 

 and cavities. 



Modify the idea of an infinite ocean of liquid into that 

 of an infinite jelly or elastic substance in which the liquid 

 is entangled, and through which it caniot penetrate 

 without violence and disruption ; and you have here a 

 model of the general insulating atmosphere. Our ocean 

 of fluid is not free and mobile like water, it is stiff and 

 entangled like jelly. 



Nevertheless bodies can move through it freely. Yes 

 bodies can, it is the liquid itself only which is entangled. 

 How we are to picture freely and naturally the motion 

 of ordinary matter through the insulating medium of 

 space it is not easy to say. It is a difficulty not fatal 

 but sensible, and due to an imperfection in our analogy. 



Insulators being like elastic partitions or impervious 

 but yielding masses, conductors are like cavities, porous 

 or spongy bodies perfectly pervious though with more or 

 less frictional resistance to the flow of liquids through 

 them. Thus, whereas bodies easily penetrable by matter 

 are impervious to electricity, bodies like metals which 

 resist entirely the passage of matter, are quite permeable 

 to electricity. It is this inversion of ordinary ideas of 

 penetrability that constitutes a small difficulty at the 

 beginning of the subject. 



However, supposing it overcome, let us think of these 

 insulated spheres and cylinders on the table connected 

 by copper wire as so many cavities and tubes in an 

 otherwise continuous elastic impervious medium which 

 surrounds them and us, and extends throughout space 

 wherever conductors are not. All, however, cavities as 

 well as the rest of the medium, are completely full of the 

 universal fluid. The fluid which is entangled in insulators 

 is free to move in conductors ; whence it follows that 

 its pressure or potential is the same in every part of a 

 conductor in which it is not flowing along. For if there 

 were any excess of pressure at any point, a flow would 

 immediately occur until it was equalized. In an insulator 

 this is by no means the case. Differences of pressure 

 are exceedingly common in insulators, and are naturally 

 accompanied by a strain of the medium. 



[Here certain electrostatic experiments were shown as 

 evidence of the strain existing at the ends of a long insu- 

 lated wire connected to a Voss machine.] 



There have been, as you know, two ancient fluid 

 theories of electricity— the one-fluid theory of Franklin, 

 and the two-fluid theory of Symmer and others. A great 

 deal is to be said for both of them within a certain range. 

 There are certainly points, many points, on which they 

 are hopelessly wrong and misleading, but it is their 

 foundation upon ideas of action at a distance that con- 

 detnfis them, it is not the fluidity. They concentrate 

 attention upon the conductors ; whereas Faraday taught 

 us to concentrate attention on the insulating medium 

 surrounding the conductors — the "dielectric " as he termed 

 it. This is the seat of all the phenomena : conductors 

 are mere breaks in it— interrupters of its continuity. 



To Faraday the space round conductors was full of 

 what he called lines of force ; and it is his main achieve- 

 ment in electrostatics to have diverted our attention from 

 the obvious and apparent to the intrinsic and essential 

 phenomena. Let us try and seize his point of view before 

 going further. It is certainly true as far as it goes, and 

 is devoid of hypothesis. 



Take the old fundamental electric experiment of rubbing 

 two bodies together, separating them, and exhibiting the 

 attraction and repulsion of a pith ball, say, and how 

 should we now describe it ? Something this way. 



Take two insulated disks of different material, one 

 metal, say, and one silk, touch them together, the contact 

 effects a transfer of electricity from the metal to the silk ; 

 rub slightly to assist the transfer, since silk is a non-con- 

 ductor, then separate. As you separate the disks the 

 medium between them is thrown into a state of strain, 

 the direction of which is mapped out by drawing a set of 

 lines, called lines of force, from one disk to the other, 

 coincident with the direction of strain at every point. As 

 Faraday remarked, the strain is as if these lines were 

 stretched elastic threads endowed with the property of 

 repelling each other as well as of shortening themselves ; in 

 other words, there is a tension along the hnes of force and a 

 pressure at right angles to them. When the disks are 

 near, and the lines short, they are mainly straight, Fig. i. 



Fig. I. — Rough diagram of the s'ate of the, medium near two oppositelj 

 charged disks when close together. 



but as the distance increases they become curved, bulging 

 away from the common axis of the two disks, and some 

 even curhng round to the back of the disk (Fig. 2), 

 until when the disks are infinitely distant as many lines 

 spring from the back of each as from its face ; and we 

 have a charged body to all intents existing in space by 

 itself. 



The state of tension existing in the medium between 

 the disks results in a tendency to bring them together 

 again, just as if they were connected by so many elastic 

 threads of no length when unstretched. The ends of the 

 lines are the so-called electrifications or charges, and the 

 lines perpetually try and shorten and shut up, so that 

 their ends may coincide and the strain be relieved. Hi 

 one of the disks touch another conducting body, some ofj 

 its lines instantly leave it and go to the body ; in other I 

 words, the charge is capable of transference, and the new i 

 body is urged toward the other disk, just as the disk wasj 

 from which it received the lines. If this new body com- \ 

 pletely surrounds the disk, it receives the whole of ^.1 

 lines, and the disk can be withdrawn perfectly free 

 inert. [Faraday's " ice pail " experiment.] 



Now take the two charged disks, facing one anot 



