Hall Effect and Specific Resistance of Evaporated Films of Silver, Copper and Iron

Specific resistance and Hall coefficient of evaporated films, 20 to 200 {μ}{μ} thick.---The specific resistances of films of silver, copper and iron were found to be from 15 to 57 times the values for the bulk metals, being respectively 24.3, 29.7 and 506 instead of 1.6, 1.8 and 8.8. The Hall coefficients of films of silver and copper, two diamagnetic metals, were respectively 41 and 11 per cent. less than the bulk values, while for the iron films the coefficient was +42.5 as compared with +6.6 for bulk iron, an increase of over 500 per cent. For the silver and copper films the Hall effect is proportional to the field throughout, but for iron films the proportionality extends only to 10,000 gausses, whereas the limit for bulk iron is 20,000 gausses. This indicates that the maximum intensity of magnetization in iron films is only half as great as that attainable in pure bulk iron.Structure of evaporated metal films.---X-ray analysis showed that films of silver and iron and probably other metals have the same crystal structure as the bulk metals, but the crystal grains are too fine to distinguish with a microscope.Effect of extreme fineness of crystal structure on certain physical properties of metals.---The increase of specific resistance is probably due to the multiplication of the number of contacts between grains. The Hall coefficient is practically independent of the structure except as it affects the magnetic condition of the crystals. The intensity of magnetization in iron crystals seems to be limited by the demagnetizing action of the poles of the crystals, which is greater the smaller the crystal grains.Method of producing uniform metallic films by evaporation.---This involves moving a heated filament uniformly over the surface to be coated, at a distance less than the mean free path in a high vacuum. The films are hard and bright and resist corrosion.