The thickness of the saturated helium film above and below the λ -point

Abstract

The properties of the static helium film covering a vertical, polished metal surface, at heights between 0∙4 and 1∙6 cm above bulk liquid and from 1∙2 to 3∙8°K, have been reinvestigated using an improved form of apparatus based on the optical method of Burge & Jackson (1951). The original calibration curve of the instrument was found to be incorrect by about 30%, and an entirely new one has been constructed in which the only assumption is that the refractive indices of film and bulk liquid are the same. At 2∙05°K the film thickness d is given by d = k / H 1/ z , where H is the height above bulk liquid, k = 3∙15 x 10 –6 cm and z = 2∙3. The value of z increases to about 2∙6 as the temperature falls to 1∙32°K , but at temperatures below 2∙05°K it varies slightly with height. The value of k varies little between 1∙2 and 3∙8°K, and it has been shown that the film is essentially the same above and below the λ -point. This confirms the theories of the film which are based primarily on van der Waals’s forces of attraction, and it seems clear that the existence of the film is not connected with the λ -phenomenon. Below the λ -point a thin layer of solid air on the metal surface increases the static film thickness enormously. Above the λ -point this phenomenon does not occur, but extreme temperature homogeneity is necessary in order that the film may form. The differences between these results and those of other investigations of the film are discussed.