20 
PROFESSORS W. E AYRTON AND J. PERRY 
desirable that the present investigation should not he thus limited, it became necessary 
to construct a new apparatus, of which the end elevation is shown in Plate 5, and the 
side elevation in Plate 4. 
Again, in the old apparatus it was necessary to open the metal cover enclosing the 
whole apparatus in order to lower, rotate, and raise plates 1 and 2. Now we consider 
that, although theoretically there was no objection to opening this case, provided it 
was shut again to screen the apparatus from exterior electrical induction before a read¬ 
ing was made, still that, as it introduced a possibility of the plates 3 and 4 being 
touched during an experiment, this opening was objectionable. In the present 
arrangement, the table A B, which carries the levelling screws l l for supporting the 
substances of which the contact differences of potential are to be measured (in our 
figure a liquid, L, and a solid plate, P, about 530 square centimetres in area), runs with 
three wheels, W, on a circular very rigid metallic horizontal railway, It; and to avoid 
lateral motion the table is kept centred by a stout iron pin, M, turning in a brass 
socket, S. The necessary vertical motion is produced in this apparatus by the plates 
3 and 4, which are attached to the upper framework with an arrangement similar to 
that employed with a parallel ruler, being raised or lowered by a vertical brass rod, rr, 
capable of rotation, so as to allow of the pin, passing through the slot, s s (shown 
in plan below), or being stopped by it when the rod is required to remain raised. The 
plates 3 and 4, which are each about 300 square centimetres in area, are supported by 
chemically clean glass rods, G, kept artificially dry by pumice-stone soaked in sulphuric 
acid, and resting in the leaden cups, U, sliding into the brass cylindrical tubes, B, with 
bayonet joints. These leaden cups may be lowered by handles, h, so as to rest on the 
brass plates 3 and 4 when the instrument is not in use, thus preventing access to B of 
the external air. The permanent adjustment for coincidence of the plane surfaces of 
the brass plates is effected by means of three screws, w, in each of the brass caps, C. 
Temporary adjustment of the horizontality of the brass plates is effected by means of a 
spirit level and the three large levelling screws, K, which support the whole apparatus, 
and the positions of which are fixed by hole, slot, and plane. 
T T are strips of brass soldered to the induction plates 3 and 4, and connected by 
means of clips to fine platinum wires leading to the insulated electrodes of the quadrant 
electrometer, and to the insulated terminals of a highly insulated short circuit key, 
not shown in the drawing. 
In addition to the whole framework being made very stout and of cherry-wood, one 
of the Japanese woods that warps least, it was further strengthened by various cross 
tie-pieces, as shown in the drawings. 
The whole apparatus, including the short circuit key and electrometer, was, to avoid 
induction from outside, enclosed in a large zinc case connected with the earth, and 
was not opened at all during one complete experiment, consisting of some ten short 
circuitings of the upper plates, reversals of the table A B, and corresponding readings 
to the right and left of the electrometer needle, 
