246 



National Resources Planning Board 



Figure 74. — Electron Diffraction Pattern of (a) Plated and (b) Stripped Metal Surface (after H. R. Xelson) 



simple piece of equipment. It is only necessary to 

 have a heated source of electrons and a high-voltage 

 supply to accelerate them in a tube. Since the electrons 

 are scattered primarily by surface atoms, while X-rays 

 are scattered deep in the metal, electron difl'raction is 

 most useful in studying surface properties. It has been 

 used, for instance, to identify various oxide coatings, 

 tarnishes, and other corrosions produced on the surfaces 

 of metals. If one wants to know the structure of an 

 electroplated surface, electron diffraction can usually 

 give the answer. Until recently it has been difl&cult to 

 determine exactly the structure of thin films because 

 X-rays pass through them so easily. Electron diffrac- 

 tion on the other hand tells us a great deal about the 

 structure of these thin films. Polished and buffed 

 surfaces are increasing in importance in industry and 

 electron diffraction provides evidence as to whether 

 such surfaces are crj'staUine or amorphous, and also as 

 to the changes in a surface as buffing progresses. These 

 are only a few of the possible applications of electron 

 diffraction in industry. Wliorever the nature of a 

 surface comes into question, use can be made of this 

 tool. 



Extreme Pressures 



A very interesting curve has been drawn showing 

 how the largest obtainable pressure has increased with 

 time. This curve has started upward rapidly in the 

 past few years; in fact, it has more than doubled in 

 about 3 years. It is now possible to study in the 

 laboratory pressures up to 1,500,000 Ib./sq. in. It 

 may be wondered why it is desirable to stuily such 

 extreme pressures, but when one realizes that enor- 

 mous pressures often occur in industry in liypoid gears, 

 ball bearings, glass cutters, and rifles, the interest is 

 understandable. Many strange phenomena occur at 

 extremely high pressures. For instance, chemical 



reactions that do Jiot take place at ordinary pressures 

 can sometimes be promoted by an increase in pressure. 

 Under high pressures it is possible to bend glass without 

 breaking it, to precipitate colloidal particles from a 

 solution, and to produce many other unusual effects. 

 Studies have been made of the penetration of liquids 

 and gases into solids. It is found, for instance, that 

 under great pressure hydrogen penetrates into steel 

 sufficiently to decrease its tensile strength bj' more 

 than one-half. A recent important industrial study has 

 concerned the effect of extremely high pressures on 

 lubricating oils. In fact, it is only because of the 

 development of lubricants working well at extreme 

 pressures that the use of hypoid gears in automobiles 

 has become possible. In view of the great number of 



FiGURK 75. — Motion of a Pelton Wheel Frozen with tlie .Aid of 

 lligh-Spced Photograjjliy lafter Harold E. Edgerton) 



