ON THE CONTACT THEORY OF VOLTAIC ACTION. 
21 
We shall now describe a complete operation to obtain the contact difference of 
potentials, between a metal and liquid, for example. Suppose the permanent adjust¬ 
ments to have been made, and the gilt plates 3 and 4 to be quite bright. The plate P 
is cleaned with emery paper that has touched no other metal, and all traces of the 
emery removed by means of a clean cloth; it is then placed on the three levelling 
screws, l, and fixed in position by hole, slot, and plane. The porcelain dish containing 
the liquid is laid in a metal one just fitting it, and on the base of which is a hole, slot, 
and plane ; this is now laid on the other levelling screws, l. 
The rod, r r, is then lowered until the disk, d d, rests on a brass plate let into the 
top of the wooden framework at the top of the instrument—that is, until the induc¬ 
tion plates 3 and 4 are in their lowest position. The levelling screws, 11, are now 
raised until a small metal ball, of a diameter of 8 millims., is in contact at three fixed 
points with the plate 4 and the plate P, or until when in contact with the plate 3 
it and its reflection in the liquid L appear to meet. To avoid any harm arising from 
possible contact of the liquid with this gauge ball, it was made of a material not acted 
on by the particular liquid under experiment. 
Before proceeding further, each pair of quadrants is in succession put to earth, the 
other pair remaining insulated in order to test for any possible leakage from the 
needles to the quadrants. Each pair of quadrants is now charged with a battery, the 
other pair being connected with the earth, in order to test for any leakage along the 
glass rods G, the small glass rods supporting the quadrants in the electrometer, or 
along the paraffined ebonite pillars of the short circuiting key. It having thus been 
ascertained that there is no leakage, the strip of metal which has been cut from 
the same sheet of metal as P itself, and temporarily attached to it by a binding screw 
soldered to P, is made quite bright with emery paper and a cloth, and its end dipped 
into the liquid L, as shown in Plate 4. The zinc case is then closed up, plates 3 and 4 
connected together, and with the earth, by means of a key (the handle of which 
was a long thin ebonite rod projecting through the zinc case), and the electrometer 
reading taken. 3 and 4 are then insulated from one another, and from the earth, and 
raised by means of the rod, r r, projecting above through the zinc case ; the table A B 
turned from below by means of a handle passing through the base of the instrument; 
3 and 4 then lowered into exactly their former position, ensured by the parallel motion 
of the supporting beam and by the limiting stop, d d. The reading of the electro¬ 
meter is now taken. Again short circuit, insulate, raise, reverse, and lower, and take 
a new electrometer reading, &c. 
Some ten readings having been thus obtained, a fresh set of experiments is always 
made with the same two substances in the following way in order to compensate for 
the error introduced by defects in parallelism of the apparatus affecting the result 
obtained from two rigid surfaces (as that of copper and zinc), differently from the 
result found with one or with two liquid surfaces under test. Instead of commencing, 
as before, with the liquid L under 3 and the plate P under 4, we start with the plate 
