The Diffraction of Electrons by a Crystal of Nickel

HE experiments which I have been asked to describe are the most recent of an investigation of the scattering of electrons by metals on which we have been engaged in the Bell Telephone Laboratories for the last seven or eight years. The investigation had its inception in a simple but significant observation. We observed some time in the year 1919 that when a beam of electrons is directed against a metal target, electrons having the same speed as those in the incident beam stream out in all directions from the bombarded area. It seemed to us at the time that these could be no other than particular electrons from the incident beam that had suffered large deflections in simple elastic encounters with single atoms of the target. The mechanism of scattering, as we pictured it, was similar to that of alpha ray scattering. There was a certain probability that an incident electron would be caught in the field of an atom, turned through a large angle, and sent on its way without loss of energy. If this were the nature of electron scattering it would be possible, we thought, to deduce from a statistical study of the deflections some information in regard to the field of the 90 TIIE DIFFRACTION OF ELECTRONS BY A CRYSTAL 91 deflecting atom. It was with these ideas in mind that the investigation was begun. What we were attempting, it will be seen, were atomic explorations similar to those of Sir Ernest Rutherford and his collaborators but explorations in which the probe should be an electron instead of an alpha particle.