THE ELECTRON— COMPTON 217 



but it encountered inconsistencies. There was not even any real 

 evidence that electricity in metals was conducted by electrons. 



Then along came Tolman with one of his brilliant ideas, sldlfuUy 

 followed by experiment. It had earUer been suggested that, whatever 

 are the carriers of electric current in metals, it should be possible to 

 centrifuge them toward the periphery of a disk if this were rotated 

 very rapidly about its axis. To be more specific, if electrons are free 

 to move in metals and if a wire connects the center and the periphery 

 of the rotating disk through lightly pressing brush contacts, electrons 

 should be thrown out of the disk at its periphery and pass back into 

 the center of the disk through the wire. It would be rather analogous 

 to a current of water driven by a centrifugal pump through a pipe 

 circuit. But all attempts to detect such currents proved futile, be- 

 cause the currents produced by the friction of the contact against the 

 periphery were far larger than the currents to bo expected from the 

 centrifuging of electrons. 



But Tolman devised two methods of giving powerful accelerations 

 to metal conductors in such manner that he was able to measure the 

 feeble electric currents that were produced as the carriers of electricity 

 in the metal wore shaken back and forth, and his calculations showed 

 that these currents were indeed of the size to be expected if the current 

 is carried by electrons. This is our direct evidence that electrons 

 carry the electric current in metals. The mechanism by which they 

 do this is now beginning to be disclosed by Slater, on the basis of an 

 apphcation of quantum mechanics and spectroscopic ideas to metals, 

 and again is an example of the refining power of the quantum theoiy 

 to succeed where older classical theory was gropingly suggestive, but 

 inadequate. 



STRUCTUKE OF THE ATOM 



Now that I come to the most basic of all the phenomena which the 

 electron has been called upon to interpret, I almost lose courage, for 

 the subject is too vast and complex for anything but encyclopaedic 

 treatment. I refer to the structure of atoms. Previous to the dis- 

 covery of the electron, Uterally nothing was known of the internal 

 structure or composition of atoms. With this discovery, however, it 

 immediately became evident that all atoms contain electrons and an 

 equivalent amount of positive electricity in some form. It was again 

 J. J. Thomson's genius which began the investigation of the inner 

 atom. This was only about 25 years ago. 



Thomson reasoned that, if X-rays were made to fall on any sub- 

 stance, the electrons in the atoms of the substance would be forced to 

 vibrate back and forth by the powerful alternating electric forces in 

 the X-ray waves. But, in thus vibrating back and forth, these elec- 

 trons would re-radiate secondary X-rays in all directions. He calcu- 

 lated just what fraction of the original X-ray energy ought to be thus 



