THE ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 
459 
in like manner constructed. By taking out from these two sets of curves the thickness 
and resistance at any particular time, the specific resistance could be calculated by the 
formula given in our previous paper. 
When the films became thin they were generally so uniform in tint that it was 
sufficient to determine the thickness half way between the two needles. 
VI. Description of the Apparatus. 
The experiments were made in a room with a south aspect, the film apparatus being 
supported in the window recess. Plate 59, fig. 1, represents a sectional elevation of 
this apparatus from front to back, taken very nearly through the centre. Plate 60, 
fig. 5, shows the same in plan, with the mirrors and the telescope of the cathetometer. 
A and B (fig. 1) are iron cylindrical cups, each 33 millims. in diameter, placed 
vertically one above the other, between the edges of which the cylindrical liquid film 
is formed. A small brass tube C, attached to A, slides with easy friction through 
another tube F, and enables the length of the film to be adjusted at will. F has a 
brass flange which carries a binding screw, and is cemented to a glass disc G, for 
closing the circular aperture at the top of the glass case HH. Through this aperture 
the cup A, with attached. tube, &c., can he removed from the case for purposes of 
cleaning. A piece of indiarubber tubing fastened to the end of the tube C passes 
through the base board of the apparatus at the point a (fig. 5), and is connected with 
a reservoir of air. 
The lower cup B is soldered to the top of a brass tube DD, which slides in a 
slightly larger tube, and can hence be removed when required. The smaller brass 
tube E was originally intended to afford a means of filling or emptying the cup from 
the outside. In the later experiments it was not used for this purpose, but, its upper 
end being stopped up with sealing wax, it merely served as a metallic connexion 
between the film and the source of electricity. 
A film, when under examination, is surrounded by two glass cases, the front and 
side faces of which are constructed of specially polished glass. The top of each is of 
ordinary plate glass, and the back of wood. The inner case HH fits tightly down 
by its wooden base upon the square ebonite platform I, by which the lower cup is 
insulated. The outer case H'H' slides down upon the inner by grooves cc, fig. 5, 
and is held firmly in its place by the hack of the inner case, and by the base board I' 
(fig. l). The object of the outer case is to prevent sudden changes of temperature or 
hygrometric state in the interior of the apparatus. 
The needles (nnn, fig. 1, shewn also in figs. 2, 3, and 5), were, in the experiments 
described in this paper, made of gold wire. They pass through small holes drilled in a 
vertical pillar of ebonite. Fine silk-covered copper wires soldered to their extremities 
make connexion with the mercury cups M, and hence with the electrometer. To 
provide against the ebonite pillar being wetted by the bursting of the films, and 
3 o 
MDCCCLXXXT. 
