221 



DISCOVERY 



Electrons and Ether 

 Waves ' 



By Sir William Bragi,', K.B.E., F.R.S. 



Sobcl Laureate ; Prolessar olPhysies In the Uniucrsilij 0/ London, 

 Uniucrsilj) College 



I PROPOSE to ask you to consider for a short time 

 one of the outstanding problems in Physics. I am 

 justified, I think, in saying that so far it has proved 

 insoluble, but for all that, it lacks neither interest nor 

 importance. It is important because it relates to very 

 fundamental things with which we are deeply con- 

 cerned, and as to its interest, it comes in many ways. 



Man's interest in radiation is naturally very old 

 indeed. The warmth of the sun, the light that it gives 

 by day, and the light of the moon and stars by night, 

 fill a first place in their importance to him. When 

 experimental science began to grow rapidly, its first 

 efforts were devoted to an attempt to unravel the laws 

 of propagation of light and heat. Among the famous 

 pioneers Newton and Huyghens represented two 

 opposing schools of thought. The former advocated a 

 corpuscular theory of light, the latter maintained that 

 Ught consisted of a wave motion. In a restricted 

 sense, the wave theory has completely triumphed ; it 

 explains the ordinary phenomena of light and especially 

 of the intricate effects which depend on interference 

 of waves with the greatest satisfaction and precision. 

 But, on a wider view of light plienomena, the victory 

 of the wave theory is not so absolute, for it is certain 

 that a great part is played by corpuscular radiations, 

 the corpuscles being the electrons of recent discovery. 

 It seems that we must admit the importance of each 

 view and, to a certain extent, we can accurately define 

 the part that each must play : but, there is one great 

 exception. There is one problem in connection with 

 the interrelations of electron waves and corpuscles 

 which seems to ridicule all our attempts to understand 

 it. If we could solve it we should have made an im- 

 mense advance, both in knowledge and in our power 

 of handling materials. We should perhaps have added 

 a new province to the realms of physical thought. And 

 it is because of this obvious importance and because 

 of our failures to find the solution that I hope j'ou will 

 be interested in looking at the question once again 

 in the light of recently acquired knowledge. 



We are going to consider the relations between the 

 energies carried by ether waves and the energy carried 

 by electrons. Let us first set down the distinctive 



' This lecture was given in May to the O.xford University 

 Junior Scientific Club, and is here reproduced by their kind 

 permission. It is published in separate form by the Oxford 

 University Press at Is. 



features of each form of radiation. As regards wave 

 radiation, we must say that the energy spreads outwards 

 and weakens as it spreads, just as a sound dies away 

 in the open air. And next we must add that all waves 

 show the extraordinary phenomenon of interference. 

 Two sets of waves can tend to destroy each other's 

 actions at certain places and times, making good such 

 losses by increased actions at other places and other 

 times. By the aid of this principle Young and Fresnel, 

 and a host of workers who have followed them, have 

 built up optical theories of great power and complete- 

 ness. Note that the characteristics of a simple wave 

 arc its length - and its amplitude ' : it has no others. 



Corpuscular radiations have been obvious to us on the 

 grand scale only since the discovery of radium and of 

 X-rays. Beside the a-rays, the projection of helium 

 atoms from the bursting atoms of radio-active sub- 

 stances, we find in the general radiation of radio-active 

 substances streams of high-speed electrons. The main 

 features of these rays which concern us now can also 

 be stated briefly : 



Electrons are to be found everywhere forming part of 

 every atom. They can be set in motion by electric 

 forces, as in the X-ray tube, or they may be expelled 

 from radio-active substances. Such radiation, like 

 light radiation, has qualities. The flying particles may 

 be more or less in number, and the speed of each can 

 fall between wide limits. In other respects it is, at 

 present, assumed that they are all like each other. We 

 have not been acquainted with electron movements 

 so long as we have been acquainted with wave motions 

 in ether. The reason is perhaps a simple one : 



An electron can only maintain a separate existence 

 if it is travelling at an immense rate, from one three- 

 hundredth of the velocity of light upwards, that is to say 

 at least 600 miles a second or thereabouts. Otherwise 

 the electron sticks to the first atom it meets. The 

 action of a powerful induction coil and space to move 

 in freely, where there are no atoms to impede it, provide 

 favourable circumstances for observation, and we have 

 only been able to realise these conditions with sufficient 

 success in more recent years. 



We now know, therefore, radiation in two forms, and 

 each is independently full of interest. But it is the 

 extraordinary connection between them that is so fasci- 

 nating and yet beats us when we try to explain it. We 

 have known for many years that there is some connec- 

 tion between waves and electrons, because light, 

 especially of short wave length, can cause a discharge 

 of negative electricity, that is to say, of electrons, from 

 substances on which it falls. This, which is known as 

 the photo-electric effect, has been carefully examined 



5 The distance between adjacent crests. 

 ' The maximum distance from the mean position attained 

 by a vibrating particle. 



