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VII. On the Thickness and Electrical Resistance of Thin Liquid Films. 
Bij A. W. Keinold, M.A., F.R.S., Professor of Physics in the Roycd Naval College, 
Greemcich, and A. W. Rucker, M.A., F.R.S., Professor of Physics in the 
Royal College of Science, London. 
Received March 10,—Read May 4, 1893. 
In a paper on the “ Limiting Thickness of Liquid Films,” presented to the Royal 
Society in 1883,"^ we proved that in the case of a large number of films the mean 
thickness of that part which was thin enough to display the black of the first order 
of Newton’s rings was 11'6 p-./r. (micromillimetres). 
The validity of the methods employed was assured by the agreement between the 
results obtained. The mean thickness of a number of black films was measured 
optically by a method which involved the assumption that the refractive index of the 
substance of the film was the same as that of a large mass of the same liquid. The 
thickness of black films was also determined by measuring their electrical resistances, 
and assuming that the specihc resistance was the same as that of the liquid in mass. 
The means of the results obtained by the two methods were 11’8 g.g. and 11’3 ju,./x. 
respectively, which were sufficiently in accord to justify the assumptions. 
While, however, there was this close agreement in the mean results, the numbers 
obtained by different experiments differed rather widely. We then expressed the 
opinion that “ the differences between the numbers given by the optical method are, 
perhaps, not much in excess of the probable error of experiment, but in the case of 
the electrical observations they far exceeded it.”t We then proceeded, in a passage 
which it is perhaps unnecessary to quote at length, to give reasons for our belief that 
“ the thickness of the black portions is really different in different films,” and added 
that “ into the cause of the variations in the thickness of different black films we do 
not now propose to enquire.” Since that date we have made a large number of new 
experiments in which the thicknesses of black films of different composition have 
been determined both by electrical and optical methods. Some of the observations 
were described to the Society in a paper “ On Black Soap Films,” which was read on 
March 1, 1890. The main object then before us was to enquire whether the rate of 
formation of the black depends on its thickness. Before the paper was published, 
fresh facts came to our knowledge which appeared to show that however this might 
be, there was an undoubted and much more clearly marked relation between the com- 
* ‘ Phil. Trans.,’ 1883, Part 2, p. 64-5. 
t Loc. cit., p. 660. 
3 T 
mdcccxciii. —a. 
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