570 
MESSRS. R. T. GLAZEBROOK AND S. SKINNER ON THE 
electromotive force of a Clark cell, and, in the second, with the comparisons between 
various cells and the conclusions to be drawn from them. 
§ 2. Theory of the Method. 
The theory of the method is very simple. A fairly constant current is passed 
through a silver voltameter and a known resistance. The value of the current is given 
by the voltameter, and from this the potential difference between the terminals of the 
resistance is found. This potential difference is compared by a potentiometer method 
with the E.M.F. of a Clark cell, and thus an absolute value is obtained for the E.M.F. 
of the cell. 
We shall describe in turn the various parts of the apparatus and the method of 
experimenting. 
§ 3. The Standard Cell. 
Lord Rayleigh left at Cambridge, in the charge of one of us (It. T. G.), three of 
the cells he constructed during the year 1883. One of these has since dried up. The 
other two, which have been carefully kept, have been compared from time to time, 
and differ very slightly in E.M.F. (see Part II.). We took one of these, which we 
call Rayleigh No. 1, as our standard. Rayleigh No. 1 is a cell of the pattern originally 
adopted by Latimer Clark. According to the data in Lord Rayleigh’s paper, “ the 
saturated solution of zinc sulphate was nearly neutral. The metallic zinc was bought 
as pure from Messrs. Hopkin and Williams. The mercurous sulphate was from the 
same source, and the metallic mercury was redistilled in the laboratory.” 
It seems probable that the cell Rayleigh No. 1, is No. 4 of Lord Rayleigh’s 
paper, though it is difficult to be quite certain of this point. In his paper, £ Phil. 
Trans.,’ 1885, Lord Rayleigh says : “ Cells (4), (8), (9), were, I think, left at Cam¬ 
bridge.” Of these it is clear (‘Phil. Trans.,’ 1 884, p. 442), that No. 4 was of the 
original pattern, the others were made at a later date. Our cell, No. 1, is of the 
pattern originally devised by Latimer Clark, while No. 2 is of the form used later 
by Lord Rayleigh. We therefore infer that our No. 1 is Lord Rayleigh’s No. 4. 
In the earlier experiments, the E.M.F. of this cell was compared directly with the 
difference of potential between the ends of the resistance. As in this procedure there 
was some slight risk of passing an appreciable current through the cell, it was 
modified. A large cell, denoted in what follows by No. 90, or the bottle cell, was 
constructed in a glass bottle, and the E.M.F. of this was determined. This cell was 
from time to time compared with the standard. 
§ 4. The Voltameters. 
Fi ve different platinum bowls were used. 
I. The large bowl employed by Lord Rayleigh, in shape approximately the 
segment of a sphere, diameter across the top 10 cms., depth 4’5 cms. 
