648 
PROFESSORS A. W. REINOLD AND A. W. RUCKER 
prism of ebonite, through which pass two straight gold wires, the so-called needles. 
The extremities of the latter are soldered to fine insulated copper wires, which pass 
through a hole in the tube, seen at N, and are connected at the top of the tube to the 
binding screws, c, c. A glass shield, cemented to the ebonite clip P, and perforated 
with holes through which the needles pass without touching the glass, is employed to 
protect the pillar which supports the needles from the spray caused by bursting of the 
films. The tube J can not only be turned about its own axis, but the hole through 
which the sheath K passes being slotted, as shown at W, fig. 3, can be moved to 
and fro parallel to itself to a limited extent. By these means the needles can be 
inserted in the film in any suitable position. Only two needles are represented in the 
figure. There were three, but as a rule two only were used, a and b are binding- 
screws for connecting the upper and lower supports of the film respectively with the 
circuit. 
On the left of figs. 1 and 3, and in fig. 2 is shown the arrangement for saturating 
the inside of the box with moisture. It consists of an endless strip of linen 0, 
passing over an ebonite roller Q, and kept stretched by an ebonite roller B below, the 
latter being weighted by a core of lead. The frame S carrying the strip can be raised 
or lowered by means of a nut T working on a screw fastened to the frame, and thus 
the linen can always be made to touch the liquid with which the bottom of the box 
is covered. The bevelled wheels seen at U, and the screw-head V, show how the 
upper roller is rotated so as to bring all parts of the linen in succession in contact with 
the liquid. The strip becomes elongated when wet, but by the nut T it can be drawn 
up to a suitable height. All the fittings connected with this part of the apparatus 
are of ebonite. 
The hair hygrometer and the thermometer are not shown in fig. 1. They are sup¬ 
ported on a single frame which is attached to the lid at Z, fig. 3. X is a plug closing 
a hole through which the liquid is introduced into the box. 
To ensure constancy of temperature the film-box was placed in the centre of a glass 
tank full of water at the temperature of the room. The tank is made of thick glass plates, 
bevelled at the edges and cemented together. It is 30 centims. high by 25 centims. 
square. In the centre of it is fitted a square glass case of the same height as the 
tank, open top and bottom, and just large enough in cross section to allow the film- 
box to slide into it down to a fixed support. When the film-box is in its place it is 
surrounded by 7‘5 centims. of water on every side except top and bottom. It was 
necessary to leave the top exposed in order to be able readily to move the needles or 
the linen strip, but the space underneath was filled up with cotton wool. By these 
means the temperature inside the film case can be maintained constant for many 
hours together. 
The principle of the method of investigation employed was the same as that 
adopted in our previous experiments. A current from a battery of 9 Leclanche 
cells was passed through a film from top to bottom, and also through a box of 
