460 
PROFESSORS A. W. REINOLD AND A. W. RUCKER OK 
the insulation of the needles from each other, and from the base of the apparatus being 
thereby impaired, a small glass shield is also supported by the pillar (figs. 1, 2, 3), 
in which holes are drilled large enough for the needles to pass through without 
touching it. 
Figs. 2 and 3 show in detail the arrangements for moving the needles from the outside 
when the inner and outer cases are in their places, and for adjusting them to them proper 
position in the film. A brass tube, g, passes through the chamber below the glass case, 
and has at its lower end a milled head, h, by which it can be rotated about its axis. A 
brass rod, f passes through it, having below a milled head, Jc, and above a pinion, e, 
which engages with the rack, d, carrying at one extremity the ebonite pillar and gold 
needles. It will thus be seen that motions of two distinct kinds can be given to the 
needles from without; they may be moved to and fro parallel to themselves by 
turning the milled head, k, which works independently of the outer tube, and they may 
be turned round a vertical axis by means of the milled head, In. In the figures the 
needles are represented at right angles to the rack, d, but they may be placed at any 
other suitable angle by turning the ebonite pillar in its socket. 
Arrangement for forming the films .—This is shown on the right hand side of figs. 
1 and 5. A rectangular portion of the ebonite base board is cut away, and communi¬ 
cation thus made between the interior of the glass case and the chamber PP beneath. 
This aperture can be closed by a wooden trap-door, shown in dotted lines in fig. 1, and 
turned up or down by a milled head, t, at the back of the case. When it is turned 
down, its edges dip into a rectangular groove filled with mercury. K is a brass rod 
with screw thread cut upon it, which serves as a rack for the pinion, worked by the 
screw head, l. It carries eccentrically at the top a circular brass platform, on which 
is placed a shallow glass dish, of diameter a little greater than that of the cylinder A, 
containing the liquid for forming the films. The position of the rod K is such that 
when the platform is raised through the aperture by the screw head, l, and rotated by 
the screw head, m, the glass dish can be brought immediately under the cylinder A, 
and a horizontal film thus formed over the mouth of the latter. This done, the dish 
is lowered through the aperture which is then closed by the trap-door. 
The object of the lower chamber PP will now be evident. It serves as a storehouse 
for the liquid, which can be thus introduced into and withdrawn from the film-chamber 
without establishing any connexion between the latter and the outward air. Its base 
is of ebonite, with holes in it for the electrometer wires to pass through. The 
cyfinder DD is in contact with no material except ebonite. 
The horizontal film mentioned above is converted into a cylinder as follows :—Air 
being forced from the reservoir, the film becomes spherical, and increases in size until 
it adheres to the edge of the lower cup B, which is about three quarters full of the 
same liquid. It is then only necessary to withdraw air until the barrel-shaped film 
becomes cylindi'ical, an adjustment which can be accurately made with the telescope. 
The needles are now made to pierce the film, an operation which can in general be 
