274 DR CARGILL G. KNOTT ON 
weather to avoid rubbing, and thereby electrifying this glass case, which during © 
the experiments had to be repeatedly removed, so that the temperature inside 
might be observed and the surfaces repolished. The upper and movable plate 
of the condenser was connected with the other pair of electrometer quadrants, 
which were put to earth and kept constantly at zero potential. In all cases 
the plates were brought into direct surface contact, and the deflection on the 
electrometer scale caused by the charge left on the msulated flask and the 
connected quadrants, when the upper plate was withdrawn to a height of five 
inches, was taken as the quantitative estimate of the difference of potential due 
to the contact of the surfaces. These opposed surfaces were polished with 
emery paper, and dusted with dry chamois skin. The polishing was effected 
manually, the surface to be polished being held for the time in one hand, and 
the emery paper in the other, and the two rubbed vigorously together for a 
quarter of a minute or so. After being thus polished the surfaces were dusted 
and reset in as short a time as possible, an interval of about fifteen seconds 
elapsing between the polishing of the second surface and the first contact of 
the two plates. 
In the first series of experiments the condenser-plates remained almost 
always in contact, being separated only when a reading was to be taken, or 
when the surfaces had to be repolished and the temperature of the water in 
the flask observed. The upper disk was then virtually at the same temperature 
as the lower. Readings were taken in groups of five at a time, the interval 
between each reading being conditioned by the swing of the electrometer 
mirror, which, under the action of the bifilar suspension, had of course to come 
to rest, or nearly so, before its dications could be of any value. After each 
group of readings the surrounding glass case was removed, the temperature of 
the cooling water observed, the surfaces repolished, and the whole arrangement 
re-adjusted precisely as before. On the whole, the five consecutive readings 
of any group were very consistent considering the difficulties besetting elec- 
trometer measurements of contact forces, and were sufficiently so in all but 
a few very exceptional cases to warrant the belief that, during the two or three 
minutes necessary to make the complete set of readings, comparatively little 
change took place on the surfaces. From theoretical considerations I was led 
to try iron and copper as likely metals to give positive results. In this I was 
not disappointed ; but the difficulty of drawing any sure conclusion from the 
indications so obtained, or in any way deciding between the claims of the 
various possible explanations which might be given to account for the facts, 
induced me, after four months experimenting, to conduct the inquiry on a 
different, and what turned out to be an improved, principle. In these earlier 
experiments it is to be particularly observed that the two surfaces were at any 
instant both at the same temperature; in the later experiments the tempera- 
