THE SCIENTIFIC WORK OF C. J. DAVISSON 791 



as an electron, and especially with an electron moving as slowly as Davis- 

 son's, the deflection commences when the flying electron is still in the outer 

 regions of the atom which it is penetrating. The deflection of the individual 

 electron and the scattering-pattern of the totality of the atoms are, there- 

 fore, conditioned not only by the nuclear field but by the fields of all the 

 electrons surrounding the nucleus. How shall one calculate the effect of all 

 these? 



This is a very considerable mathematical problem, and Davisson simpli- 

 fied it to the utmost by converting the atomic electrons into spherical shells 

 of continuous negative charge centered at the nucleus. The simplest con- 

 ceivable case — not to be identified with that of nickel — is that of a nucleus 

 surrounded by a single spherical shell having a total negative charge equal 

 in magnitude to the positive charge of the nucleus itself. Within the shell 

 the field is the pure nuclear field, the same as though the shell were not 

 there at all; outside of the shell there is no field at all. This is what Davis- 

 son called a ''limited field." Calculation showed that the scattering-pattern 

 of such a system would have a peak in the direction d = 0°, so long as the 

 speed of the electrons did not exceed a certain ceiling- value ! And there was 

 more: "the main features of the scattering-patterns (Davisson said "distri- 

 bution-curves") for nickel, including the lateral maximum of variable posi- 

 tion, are to be expected if the nickel atom has its electrons arranged in two 

 shells." 



Nickel in fact is too complicated an atom to be represented, even in the 

 most daring allowable approximation, as a nucleus surrounded by a single 

 shell; two shells indeed seem insufficient, but the fact that a two-shell theory 

 leads in the right direction is a significant one. Magnesium might reason- 

 bly be approximated by a single-shell model; Davisson experimented on this 

 metal, and published (in 1923) scattering-patterns which lent themselves 

 well to his interpretation. He measured scattering-patterns of platinum also, 

 and these as to be expected are much more wrinkled with peaks and valleys; 

 the task of making calculations for the platinum atom with its 78 electrons 

 was too great. 



Nickel continued to be Davisson*s favorite metal, and four years later 

 (1925) his study of its polycrystalline scattering-pattern was still in prog- 

 ress. In April of that year occurred an accident, of which I quote his own 

 description from Physical Review of December 1927. "During the course of 

 his work a liquid-air bottle exploded at a time when the target was at a 

 high temperature; the experimental tube was broken, and the target heavily 

 oxidized by the in-rushing air. The oxide was eventually reduced and a 

 layer of the target removed by vaporization, but only after prolonged heat- 

 ing at various high temperatures in hydrogen and in vacuum. When the 



