September 4, 19 19] 



NATURE 



17 



amenable to experiment, than wc might have hoped. 

 Nevertheless, the aether is needed for an) clear con- 

 ception of potential energy, for any explanation of 

 elasticity, for any physical idea of the forces which 

 unite and hold together the discrete particles of 

 matter whether by gravitation or cohesion or electric 

 or magnetic attraction, as well as for any reasonable 

 understanding of what is meant by the velocity of 

 light. Let us Iry to realise the position beforehand ; 

 for we shall be handicapped in the progress of our 

 knowledge of the relation between matter and ather 

 until these fundamental things are settled, and until 

 everyone agrees that the aether has a real existence. 

 I want people generally to admit that the aether is 

 itself stationary as regards locomotion, and that it 

 is the seat" of all potential energy ; and further, at 

 least as a surmise, that it is the medium out of 

 which matter is probably made, and in which matter 

 is perpetually moving by reason of its fundamental 

 property called inertia — a property the full explana- 

 tion of which must, I expect, ultimately be relegated 

 to and considered as a property derived from the 

 aether itself. 



I call this lecture "/Ether and Matter," but I might 

 equally well have called it " Inertia," for that is the 

 main theme with which I have to deal — at least, i'n 

 this first part. 



Is there anything else besides matter which pos- 

 sesses or seems to possess inertia? Faraday dis- 

 covered that an electric current had a property "which 

 bore some analogy to inertia, a property' clearly 

 depending on its magnetic field. Every current, even 

 a convection current, is necessarily surrounded by 

 lines of magnetic force, and when the magnetic field 

 is intense the current behaves as if it had consider- 

 able inertia. Faraday at first called the effect "the 

 extra current." Maxwell called it "self-induction." 

 The latter is the better name. 



To show it, I start a current in a circuit containing 

 a stout ring of laterally subdivided iron round which 

 the current-conveying wire is wound, and I put in 

 circuit an instrument which only responds when the 

 current has risen to nearly its full strength. A 

 current usually rises what is called instantaneously, 

 but here there is a very noticable delay between 

 pressing down the key 'and the response of the 

 instrument. The lag shown is onlv a second 

 or two, but with care I can adjust it "until it is a 

 quarter of a minute. Such delay or lag in estab- 

 lishing a current would be fatal to electric tele- 

 graphy. In practice the delay is reduced to a mini- 

 mum by using its early values, and the actual response 

 is exceedingly quick. ' Still, the law of rise of current 

 is quite definite; there is no exception, it is onlv a 

 question of degree; and the law is' the same as that 

 appropriate to the pulling of a' barge on a canal. A 

 barge gets up speed slowly, at a rate depending on 

 its mass or inertia, and it ultimately attains a steady 

 speed when the resistance balances the pull. 



That is exactly the case of a steady current obeying 

 Ohm's law; the E.M.F. is balanced by the re'sist- 

 ance, the propelling force is zero, and the current flows 

 by what we may call its own inertia — its own 

 momentum. 



To stop the current you must either increase the 

 resistance or suspend the propelling force. If vou 

 iriterpose an obstacle suddenly, the motion stops with 

 violence — a collision in the case of a train or barge, 

 a flash in the case of electric current. This is what 

 Faraday called "the extra current at break"; and if 

 you are holding the wires in your hand when a 

 current is suddenly broken in a circuit of large self- 

 ' induction, you may get a nasty shock. 



NO. 2601. VOL. 104] 



If you could abolish electric resistance, a current 

 would go on for ever without propelling force. 



An amazing experiment has been made' by Kamer- 

 lingh Onnes at I^yden, who first cooled a rnetal ring 

 down to within 4° of -Absolute zero by means of liquid 

 helium, and then started a current through it by a 

 momentary magnetic impulse. Instead of stopping 

 in a minute fraction of a second, as usual, the current 

 went on and on, not for seconds, but for days. In 

 four days it had fallen to half-strength, and there 

 were traces of it a week later. A most suggestive 

 experiment as to the nature of metallic conduction, 

 as well as a demonstration of the fly-v^-heel-like 

 momentum of an electric current ! 



This electromagnetic analogue to mechanical 

 momentum or inertia is explicable (or supposed to be 

 explicable) in terms of the magnetic field surrounding 

 the current, i.e. really (as I thmk) in terms of a pro- 

 perty of the aether of space. It exactly simulates 

 inertia; but is it an imitation or is it the same 

 thing? Can it be said that an electric charge pos- 

 sesses inertia in its own right, and retains it always, 

 as matter does, whether it be moving or whether it 

 be stationary? 



The question was brilliantly answered by your 

 professor of natural philosophy, Sir J. J. Thomson, 

 so long ago as 1881. He calculated the inertia or 

 quasi " mass " of an electric charge e on a sphere 



of radius a, and showed that it was m= — — . 



The n need not be attended to now, though it is 

 really the most important of all — being a great 

 aethereal constant of utterly unknown value ° — but for 

 oun present purpose the ji merely signifies that the e 

 must be measured in electromagnetic, not electro- 

 static, measure when the formula is interpreted 

 numerically with /U=i. 



At the date 188 1 this expression for true electric 

 inertia, though an interesting result, seemed too 

 absurdly small to have any practical significance. 

 Take a sphere like a football, 20 cm. or 8 in. in 

 diameter ; charge it until it is ready to give more 

 than an inch spark, say up to 60,000 volts; then 

 calculate the inertia or equivalent mass corresponding 

 with the charge. If I have done the arithmetic right, 

 it comes out one-third of a millionth of a millionth 

 of a milligram (3X10-'°). Absurdly small! Yes, but 

 not zero. .And whenever a quantity is not nothing, 

 there is no telling what importance may not have to 

 be attached to it sooner or later. Nothing real can 

 be so small as to be really negligible in the long run 

 as knowledge progresses. Something at present un- 

 foreseen may bring it into prominence. So it has 

 turned out in this case. The infinitesimal result of 

 nearly forty years ago to-day dominates the horizon. 

 It was in some sort the dawn of a new era in physics. 



Consider it further. Clearly the inertia depends, 

 not on the charge onlv, but on its concentration. 

 The radius of the sphere occurs in the denominator 

 of the expression. The same charge on a sphere 

 2 cm. in diameter would have ten times the inertia ; 

 on a sohere as small as an atom the inertia would 

 be a hundred million times bigger still. But then 

 even that is small ; moreover, an atom could scarcely 

 be expected to hold such a charge. Nevertheless, 

 allowing onlv a reasonable potential, it might seem 

 that atomic inertia could be sensibly increased by an 

 electric charge. But, no; even on a sphere as small 

 as an atom the concentration turns out insufficient; 

 the effect is still excessively minute. Yet as electric 

 inertia at given potential depends on linear dimen- 



8 1 havt- Kne«se<1 that it is a density of lol' gratns per c.c.-^4ff. See 

 Ether of Spa e," Appendix 2 ; also the Phil. Mag. for April, 1907. 



■The 



