484 
PROFESSORS A. W. REINOLD AND A. TV. RUCKER ON 
Column III. gives similar numbers calculated with the thicknesses deduced from 
Newton’s rings instead of those given by the corrected table. 
Table IX. 
I. 
II. 
III. 
15-48 
1-009 
1-006 
14-10 
1-001 
•998 
12-60 
1-003 
1-008 
11-19 
•997 
1-006 
9'64 
•992 
•999 
8-41 
•995 
1-015 
7-10 
•996 
•997 
5-77 
1-004 
•997 
4-54 
1-016 
1=007 
In the last column of this table there is no regular increase or decrease in the 
value of the specific resistance and the largest numbers are those which correspond to 
medium thicknesses. A comparison therefore of the two tables seems to place it 
beyond question that a soap film thicker than 3'74xl0 -5 centims. obeys Ohm’s law 
at all events to within one per cent. 
The results of the experiments may be summed up as follows :— 
(1.) It is difficult to form a soap film under conditions such as to preclude a slight 
evaporation or absorption of water. The more nearly such conditions are attained the 
more closely does the specific resistance of the film agree with that of the liquid in 
mass. The mean specific resistance of six films observed at a real thickness of 
7'48 X 10~ 5 centims. under the most favourable circumstances agreed with that of the 
liquid to 1'8 per cent. 
(2.) The same films obeyed Ohm’s law with great accuracy and much better than 
the others. This was tested between thicknesses of 13'3 X 10~ 5 and 3'74 X 10 -5 centims. 
Within these limits the specific resistance never differed from its mean value by more 
than 1'5 per cent., and showed no tendency to increase or decrease regularly as the 
thickness diminished. 
We may therefore conclude that the specific resistance of a soap film thicker than 
3'74X10“ 5 is independent of the thickness and is equal to that of the liquid from 
which it is formed. 
The experiments afford no indication of an approach to a thickness equal to the 
diameter of the sphere of molecular attraction, but if Quincke’s result be correct they 
enable us to determine a superior limit to the difference between the specific resistance 
of the surface and interior of the liquid respectively. 
Let us suppose that the thickness of the film (T) is greater than twice the length of 
the radius of molecular attraction (to). Let It, p, and r be the apparent specific resist- 
