THE DIFFRACTION OF ELECTRONS BY A CRYSTAL 



93 



elegance of its methods and because of its remarkable facility in 

 accounting for various of the inhibitions with which the radiating atom 

 is afflicted. We have been prepared by these successes to view with 

 not too great surprise — or alarm — evidence for the wave nature of 

 phenomena involving freely moving electrons. And any reluctance 

 we may feel in treating electron scattering as a wave phenomenon is 

 apt to be dispelled when we find that the value calculated for the 

 wave-length of the equivalent radiation is in acceptable agreement 

 with that which L. de Broglie assigned to the waves which he associated 

 with a freely moving particle — that is to say, the value himv (Planck's 

 constant divided by the momentum of the particle). 



In this account of the experiments I will describe the general method 

 of the measurements and the general character of the results rather 

 than attempt to go into these matters in detail. 



Nickel forms crystals of the face centered cubic type. In Fig. 1 

 (a) the crystal which we had at our disposal is represented by a block 

 of unit cubes of this type. 



a h c 



Fig. 1 — Diagrams of nickel lattice, of cut lattice, and of lattice with 

 incident and scattered beams 



Our first step in preparing the crystal for bombardment was to 

 cut through this structure at right angles to one of the cube diagonals. 

 The appearance of the crystal after the cut was made, and the corner 

 of the cube removed, is indicated in Fig. 1 {b). It is this newly formed 

 triangular surface that was exposed to electron bombardment. The 

 bombardment was at normal incidence as indicated in Fig. 1 (c). 

 We are to think of electrons raining down normally upon this triangular 

 surface, and of some of these emerging from the crystal without loss 

 of energy, and proceeding from it in various directions. 



What is measured is the current density of these full speed scattered 

 electrons as a function of direction and of bombarding potential. 

 The way in which the measurements are made is illustrated in 

 Fig. 2. The electrons proceeding in a given direction from the crystal 



