214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



tion of atomic structure and spectra that the magnetic character of 

 the electron has, witliin the last dozen years, been put on a well- 

 established basis. 



The other thing we know is perhaps the most unexpected of all the 

 electron's properties — it behaves like a wave when it collides mth 

 other objects. Davisson and Germer discovered this in the Bell 

 Laboratories, while examining the way in which a beam of electrons, 

 incident on a solid surface, w^as scattered or reflected by it. They 

 found, if the surface were crystalline, that the electrons were scattered 

 just like diffracted X-rays, but that, unlike X-rays, the wave length 

 of an electron is not fixed but varies inversely as its speed. J. J. 

 Thomson's son, G. P. Thomson, has made very illuminating studies 

 of tliis phenomenon, which is the inverse of the Compton effect; 

 together they have given physicists two mottoes: "Particles behave 

 like waves and waves behave like particles" and "Here's to the elec- 

 tron; long may she wave." One of the triumphs of the new wave 

 mechanics (a brand of quantum mechanics) is that it offers a medium 

 of explanation of these strange phenomena. But my subject of the 

 electron is too long to lot me attempt a digression on wave 

 mechanics. 



SIGNIFICANCE OF THE ELECTRON CONCEPT 



With this skctcli of the electron itself before us, let us turn to some 

 of the more important directions in which the electron has given us an 

 interpretation of the phj'^sical universe generally. Immediately were 

 explained the phenomena of electrolysis and of ionization generally, 

 for ions were simply atoms or groups of atoms which had gained or lost 

 one or more electrons. Primary chemical forces were explained as the 

 electrostatic attraction between atomic groups which, respectively, 

 contained an excess or a deficiency of electrons. (The more refined 

 interpretation of chemical forces within the past half-dozen years, by 

 Pauling and Slater, has been based upon the quantum theory of 

 atomic structure.) 



The three types of rays from radioactive substances were interpreted: 

 alpha rays as helium atoms which had lost two electrons; beta rays as 

 electrons; and gamma rays as X-ray-like radiations. In fact, Bec- 

 querel showed the magnetic deflection of beta rays in the same year, 

 1897, that Thomson showed the magnetic deflection of cathode rays 

 and interpreted them as electrons. 



For many years two unexplained phenomena had been studied in 

 metals. \Mien highly heated or when illuminated by ultra\4olet light, 

 metals had been shown to emit negative electricity. It was the work 

 of but a year, after the discovery of the electron, for J. J. Thomson 

 and his pupils to show that both these phenomena consist in the 

 emission of electrons. But by what mechanisms are they thus emitted? 



