650 
PROFESSORS A. W. REINOLD AND A. W. RUCKER 
used, the third being bent on one side and not touching the films. On one occasion, 
however, the first and third needles were used, the middle one being bent back. 
A mass of liquid having the form of a distorted circle, with its longer axis vertical, 
formed around each needle at the point where it entered the film. The horizontal 
wire of the cathetometer telescope was made to touch this circle at the top and bottom, 
and the mean reading was taken as that proper to the needle. The ratio between the 
mean diameter of these liquid masses and their distance apart was a necessary datum 
in applying a correction to the direct results. This correction was necessitated by the 
fact that the equipotential lines, which in the undisturbed film are horizontal circles, 
are distorted in the neighbourhood of the liquid masses formed by the insertion of the 
needles. 
If a be the mean radius of these circles of liquid, 
b their distance apart, 
then calculation shows that the observed resistance of the film between the needles 
must be increased by the following percentages:— 
If - = 6, 7, 8, 9, 10 
CL 
percentage to be added = 6T, 4'3, 3‘4, 2‘6, 2'2. 
The films, although they were all made from the same solution and thinned under 
apparently identical conditions, behaved very differently one from another. Sometimes 
a ring of black was seen to form a few minutes after the film was blown, and to extend 
rapidly downwards. At other times half an hour elapsed before any black appeared. 
The passage of the electric current has a considerable effect in retarding the initial 
formation of the black, and sometimes prevents it altogether. When the black is once 
formed, the passage of the current appears sometimes to check its growth, but this 
effect is not always observed. As a rule the circuit was not completed until the film 
was in a condition suitable for electrical measurements, or, in other words, until the 
black had extended to a distance of l'b or 2 millims. below the second needle. Few 
films reached this stage, and we considered ourselves fortunate if in a day’s work we 
succeeded in making trustworthy observations on a single film. If the black reached 
the second needle it not unfrequently continued to spread far below. On three or 
four occasions the film became black from top to bottom, a distance of 34 millims. 
The phenomenon of a cylindrical soap film, 32’5 millims. in diameter and 34 millims. 
long, black throughout its entire area, is a very remarkable one. Under these circum¬ 
stances, so little light is reflected from any part of the film, that it is difficult to say at 
first sight whether a film is present or not. 
The following table (Table I.) contains the results of our observations of 13 films. 
No results have been omitted from the table excepting such as we knew to be affected 
with error owing to defective insulation of the needles or other causes. 
