466 
PROFESSORS A. W. REIROLD ART) A. W. RUCKER OR 
stout copper wires passing through, were inserted in succession in the mercury cups 
(M 1,1), and the deflections written down in order, the time of each observation, 
given by a chronometer, being noted by the side. 
It was generally found that the needles were of slightly different potentials even 
when no current was passing, and it was necessary to correct for this which would 
affect the apparent resistance of the film. This difference was measured after the film 
was formed and before the current was allowed to traverse it. It was generally small, 
and sometimes vanished, but as a rule caused a deflection of from 1 to 5 divisions of 
the scale. Frequently its direction was constant for the same film, and even for the 
same day, but, as its magnitude could not be relied on to remain unaltered during the 
life of a film, the current was interrupted at intervals of 5 or 6 minutes and the 
condition of the needles examined. The deflections observed were added to or 
subtracted from those obtained for the needles when the current was passing, according 
to their direction. 
The same phenomena were observed with steel and platinum needles in an augmented 
degree, and were less under control. It was always found that the difference of 
potential between needles 1 and 3 was equal to the sum of the differences between 
1 and 2, and 2 and 3. This difference was not altered in any constant manner by the 
passage of the current. Sometimes during the life of a film and with the current 
continuously passing it increased, sometimes it diminished, and again at other times 
showed no signs of alteration. But in what direction it would change, if at all, could 
not be predicted. 
Polarisation so-called has nothing to do with the phenomenon, since its direction 
and magnitude do not depend upon the direction or continuance of the current. The 
cause is to be found in the difference of chemical condition of the needles produced by 
the action of the liquid quite independently of the current. The needles are affected 
to a different extent, and thus a difference of potential is set up between them. That 
this is the cause of the phenomenon was shown as follows : Two steel needles, not 
specially cleaned, were inserted in a film, and their extremities connected by wires with 
the electrometer. The deflection was +6. The lower needle was then passed through 
the fingers, and the deflection rose to +9. The lower needle was then washed, rubbed 
with emery paper, washed again, and connected with the electrometer, which now 
showed a deflection of —3. Thus by cleaning the needles unequally, either could 
be made to be of a higher potential than the other. With gold wires these changes 
occurred to a less degree than with either steel or platinum, and hence gold was 
the metal employed in the latest experiments. In these it was usual each morning to 
wash the gold needles in boiling nitric acid and then in distilled water. But even 
when the greatest care was taken in cleaning the needles, it was impossible to ensure 
their being in such a state as to exhibit when immersed in the film no difference of 
potential. 
As a specimen of the observations made in measuring the resistance of a film, we 
