396 



NATURE 



[January 



;9i8 



To get either aberration or Doppler effect the receiver 

 must move relatively to the source ; to get the Fizeau 

 drift there must be a material medium transmitting the 

 light, and that medium must be in motion with respect 

 to both source and receiver. 



We must admit, however, that if the aether is to be 

 sustained as a reality, some way out of the contradic- 

 tion of the two experiments first cited must be found. 

 Such a way out was suggested by G. F. FitzGerald, and 

 shortly afterwards independently by Prof. H. A. Lorentz. 

 It consists in supposing that the shape of bodies is 

 slightly dependent on their motion, so that a sphere 

 moving through the sether in the direction of its polar 

 axis becomes an oblate spheroid with a contracted axis, 

 or a slightly swollen equator, or both. Such a change 

 of shape, if applicable to all matter without exception, 

 would be, ordinarily speaking, undiscoverable, but 

 would account for the negative result of the Michelson 

 experiment without any appeal to the principle of 

 relativity or any abandonment of the aether of space ; 

 for the to-and-fro journey along the line of motion 

 could then be considered shortened by the requisite 

 amount, so that the time taken by light to travel in 

 what for brevity we may call the axial direction 

 (nothing to do with the axis of the earth) need be no 

 longer than that taken to travel equatorially, in spite 

 of its having to go in one case against and with the 

 stream, and in the other case across it. 



Thus with this special hypothesis the Michelson- 

 Morley observation would be justified, even though the 

 aether were streaming at full speed past the earth, no 

 part of it being carried along with that body, entirely 

 in accordance with the first experiment above cited. 

 This would have the incidental advantage of rendering 

 the theory of Bradleyan aberration quite simple and 

 straightforward, and it would help us to begin to 

 understand the relationship between aether and matter. 

 The amount of longitudinal contraction necessary is 

 verv small ; the two-hundred-millionth part of the 

 relevant dimension would suffice, a fraction correspond- 

 ing with only 2| inches in the diameter of the earth ; and 

 Lorentz showed that on the electrical theory of matter 

 such a contraction was quantitatively to be expected,^ 



The Electrical Theory of Matter. 



The electrical theory of matter took its rise about 

 1881 in some brilliant work of Sir J. J. Thomson, who 

 showed that an electrical charge conferred on the body 

 possessing it a slight extra inertia in excess of its 

 ordinary mass. 



The electric inertia thus gained by a sphere of radius 

 a charged with quantity e was 



3« 

 though this, when interpreted in micrograms, seemed 

 hopelessly too small for any possibility of observation. 

 The extra, or' electrical, inertia was due to the mag- 

 netic field excited by the motion of the charge, and was 

 of the nature of self-induction ; it reacted against 

 acceleration or any change of velocity quite in accord- 

 ance with Lenz's law. The magnitude of this inertia 

 depends on the concentration of the lines of force, or, 

 as we may express it, on the potential of the charge, 



2 In my British Association aHdress "Continui'y" I indicate a preferenci? 

 for a slightly modified chanee of this kind (see pp. 58 and 11 1), whereby the 

 volume of a moving spherical unit remains unchanged, the polar axis shortening 



{^'-2) , while the two equatorial a.xes,-i.e. those perpendicular to the 



motion, lengthen fi-^,} ■ This does all that is necessary, and evades 



some difficulties. It is, on the whole, sustained by some experiments of 

 Bucherer. 



Viz. an amount 



NO. 2516, VOL. 100] 



and is proportional to its potential energy. The poicn- 

 tial is e/Ka; the energy is half the charge x the potential ; 

 so the expression for the inertia may be written as 

 the static energy of the charge multiplied by 4/31-, 

 where c is the velocity of light. Hence the obvious 

 smallness of the result.^ 



Some time later, viz. in J887, Mr. Oliver Heavi- 

 side calculated that this electric inertia was not pre- 

 cisely constant, but must be a function of speed, and 

 gave an expression for it at any velocity, incidentally 

 showing that it tended asymptotically to an infinite 

 value at the velocity of light. 



Then Sir Joseph Larmor showed that the FitzGerald- 

 Lorentz contraction corresponds with this extra inertia, 

 by an increased concentration of the electric lines of 

 force to the equator of a moving sphere, when by 

 reason of motion it becomes deformed into an oblate 

 spheroid. 



All this, however interesting, seemed rather academic 

 and without probable realisation in practice, until in 

 1899 Sir J. J. Thomson isolated the unit electric charge 

 and discovered that it could exist apart from matter, 

 and was of excessively minute bulk even when com- 

 pared with a single atom. 



The apparently insignificant expression, Zjxe^j^a, now 

 came into prominence, for the small size of an elec- 

 tron would mean excessive concentration of the lines 

 close to the centre of force, and therefore a perceptible 

 amount of inertia,- even though the charge itself were 

 small. The inertia of electrons was actually measured 

 by ingenious vacuum experiments. 



The inertia of light-emitting particles was also 

 measurable, by aid of the Zeeman effect, and was found 

 to be the same ; and many other- measurements of 

 electric mass were made and found consistent. 



Later, as we know, the speed of extra quick-moving 

 electrons was measured, and their predicted extra 

 inertia at high speed was verified and found to be 

 correctly accounted for by electrical theory, on the 

 assumption that their whole mass was electrical.* 



Hence the speculation became reasonable that pos- 

 sibly there was no inertia in existence other than 

 electric inertia, and that the electromagnetic pheno- 

 menon with which we had been familiar ever since 

 Faradav and Maxwell, and had known for a long time 

 as self-induction, was truly the basis of all inertia, and 

 might be held to account for, and partly to explain, the 

 most fundamental property of matter. 



Thereupon arose various semi-astronomical specula- 

 tions as to the nature or structure of an atom, the most 

 probable of which at the present day assumes a central 

 positively charg-ed nucleus, of possibly _ complicated 

 structure, surrounded by an equal opposite group of 

 negative electrons revolving with intense rapiditv in 

 regular orbits and subject to various known kinds of 

 perturbation, the number of electrons per atom in anv 

 given instance being determined by the numerical posi- 

 tion of the substance in the chemical series of elements. 



Assuming, then, that the familiar mechanical inertia 

 of all matter is wholly electrical, we may summarise 

 results by saying that when stationary in the aether 

 its mass is the sum of terms like 



ma = 2ixe^l2,0', 

 but that when moving with velocity v, bearing a cer- 



3 For example, a sphere 40 centimetres in diameter, charged to a potential 

 of, say, 300,000 volts, would have an electrostatic energy of ten rnillion ergs, 



and an electrical inertia, or extra mass due to its charge, of ^^-5^ — ^°' = x 10 "■» 



gXiC^y 7 

 gram, or the seventy-thousand-millionth part of a milligram. 



■* See Sir J. J. Thomson's interpretation of Kaufm^nn's results, as given. 

 for instance, in " Conduction of Electricity through Gases," p. 535 : or in my 

 book on " Electrons," p. 134. 



