22 
PROFESSORS W. E. AYRTON AND J. PERRY 
under 3 and the liquid under 4, and readjust, by means of the levelling screws l, the 
heights of the surfaces, until their distance from the plates 3 and 4 is, as before, 
8 millims. We then short circuit, insulate, raise, reverse, and lower, and take exactly 
as many readings as before ; and the mean of the two sets of readings, obtained with 
the two modes of levelling, is regarded as the result of the particular experiment. 
III. 
. The results we have already obtained in this present investigation group themselves 
under three heads :— 
1st. The contact difference of potentials of metals and liquids at the same tem¬ 
perature. 
2nd. The contact difference of potentials of metals and liquids when one of the sub¬ 
stances is at a different temperature from the other in contact with it; for example, 
mercury at 20° 0. in contact with mercury at 40° C. 
3rd. The contact difference of potentials of carbon and of platinum with water, and 
with weak and strong sulphuric acid. 
But those contained under head No. 1 are alone contained in the present investiga¬ 
tion. The remainder of our completed experiments we desire to have the honour of 
submitting on a subsequent occasion to the Royal Society. 
A fourth very important division of the subject, namely, the measurement of the 
contact difference of potential of substances in other gases than air, it has been our 
intention for some time to take in hand; but, although we commenced the working 
drawings of the apparatus necessary for that investigation at the beginning of 1877, 
we have not yet had an opportunity of constructing it. 
The numbers which follow in the accompanying tables are those to which alone we 
attach importance, but it need hardly be said that, in such delicate experiments as the 
present, much time has to be spent in obtaining measurements which are afterwards 
found out to be wrong for a variety of reasons. All such we have not included in the 
following tables ; and this therefore explains why, in some cases, one measurement 
only is apparently the result of a whole day’s work. 
In our previous papers Nos. I. and II. we used as our standard electro-motive force 
that of a Latimer Clark’s standard mercurous-sulphate cell, since that was the most 
constant galvanic cel] known to us; and we found experimentally, as Sir William 
Thomson previously ascertained, that zinc had to copper a contact difference of poten¬ 
tials Off 5 volt. Now, although Mr. Latimer Clark’s cell is a very constant 
element, the difference of potentials of zinc and copper is even more constant; we 
have, therefore, in the present investigation compared all our measurements with this 
standard, which we have taken to be equal to 0ff5 volt. 
