DEMONSTRATION OF ELECTRON WAVES 557 



Spacing between planes than any other — a greater value of the constant 

 d. The first order ring due to these planes is the smallest in the pat- 

 tern ; the first order rings due to other arrangements follow in the order 

 of decreasing values of d. These first order rings plus their companions 

 of higher orders constitute the complete pattern of the aggregate. 

 From the sequence of ring diameters, one infers the arrangement of the 

 atoms in the crystal; from the scale of the pattern, the ratio of crystal 

 spacing or "constant" to radiation wave-length; from the relative 

 intensities of the rings, the way in which the scattering power of the 

 atom varies with angle. The ring pattern is thus a storehouse of 

 information concerning both crystal and wave. 



Patterns of this type for electrons were obtained first by G. P. 

 Thomson ; it was thus, in fact, that Thomson made his demonstration 

 of electron waves. A great many such patterns have since been ob- 

 tained and studied. Two beautiful examples by Wierl are reproduced 

 in Fig. 3. The one on the left records the diffraction of a beam of 



Ag CCI4 



Fig. 3 — On left — diffraction pattern produced by transmission of 45-kv. electrons 

 through thin silver foil — by R. Wierl. On right — -pattern obtained by transmission 

 of 36-kv. electrons through CClj vapor — -by R. Wierl. 



36 kv. electrons by a thin film of polycrystalline silver, the one on the 

 right is for 45 kv. electrons diffracted by carbon tetrachloride vapor. 

 The form of the pattern for silver — the particular sequence of ring 

 diameters which it displays — is characteristic of the so-called face 

 centered cubic arrangement of atoms, and agrees with the pattern 

 similarly obtained with X-rays. The scale of the pattern with other 

 data of the experiment, including the scale factor of the silver crystal, 

 yields an "observed" wave-length of 36 kv. electrons in agreement 

 with the value computed from the de Broglie formula. 



The pattern for carbon tetrachloride is bracketed with that for 

 silver because the chlorine atoms in this molecule have the same ar- 



