THE ELECTRON— COMPTON 209 



and days through the tapering edge of a Gillette safety razor blade, 

 but without finding any refraction. If he had happened to try the 

 edge of a crystal instead of the edge of the razor blade, he would 

 undoubtedly have discovered the peculiar diffraction of X-rays in 

 passing through crystals, discovered a couple of years later by Laue, 

 Friederich and Knipping and developed by father and son, W. H. 

 and W. L. Bragg, which proved both the wave nature of X-rays and 

 the atomic lattice structure of crystals. If Rontgcn's discovery of 

 X-rays was an accident, then I suppose Richardson's failure to dis- 

 cover diffraction of X-rays was a negative accident. I often wonder 

 how many important negative accidents slip past us week by week! 

 But to get back on the subject of the electron: it was the cathode 

 rays, which produce the X-rays, which finally turned out to be elec- 

 trons traveling at high speeds. These cathode rays had been observed 

 to shoot out in straight lines from the surfaces of cathodes in rarefied 

 gases through which electric currents were forced by high voltage. 

 Objects which they struck became luminous with fluorescent light, 

 and objects in their paths cast shadows. But their true nature was 

 disclosed when a magnet was placed near the discharge tube, for then 

 their paths were curved in a direction showing that cathode rays were 

 negatively charged. By measuring this curvature produced by a 

 magnetic field of known strength, and making a pretty sure assumption 

 that the kinetic energy of these rays was determined by the voltage 

 applied to the tube, J. J. Thomson in 1897 first showed that cathode 

 rays are negatively charged particles with a ratio of charge to mass 

 nearly 2,000 times that of hydrogen. He furthermore showed that 

 these particles are of the same type, as regards ratio of charge to mass, 

 from whatever gas or cathode material they are produced. He therefore 

 announced these particles, which he called "corpuscles," to be universal 

 constituents of all substances. Thus was the electron discovered. 



MASS AND CHARGE OF THE ELECTRON 



Quick and fast came experiments of ingenious design to study the 

 electrons more accurately. They were pulled this way and that by 

 electric and magnetic fields. They were caught in miniature metal 

 fly-traps, called Faraday cages, to measure their charge and kinetic 

 energy. They were detected in their paths electrically, or by photo- 

 graphic plates or by fluorescence. Continually refined from that day 

 to this, we now know that an electron has a ratio of charge to mass 

 which is about 1,842 times the similar ratio for a hydrogen atomic ion. 



It was also very desirable to Imow separately the charge and the 

 mass of an electron, and not just the ratio between these quantities. 

 So an even more interesting lot of experiments has been carried on 

 to measure the electron's charge. They were begun in about 1900 



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