420 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
from the readings of the thermometer placed in it. Recent results of KAHLE agree 
with our conclusions, and show that, in the H form of cell, the lag in the change of 
the electromotive force with change of temperature is avoided, owing to the fact that 
the liquid at both electrodes, being in contact with crystals of zine sulphate, is always 
in a state of concentration. We have, after the conclusion of our experiments, set up 
a, few cells according to the instructions given in the Third Memorandum of the 
Board of Trade. When proper precautions are taken to ensure equality of tempera- 
ture, these cells agree within about one part in ten thousand with each other and 
our standard cell. Taking account of the determination by GLAzEBROOK and 
Skinner of the electromotive force of a Clark cell—1°4342 at 15° C.—and of the 
slightly lower value obtained by Kantus, we have adepted 1°4340 as the value of our 
standard cell in terms of the international volt. As temperature coefficient, we have 
adopted that given by Kau rn (see footnote on preceding page). 
Fig. 3. 
5 
Cupboard Clark Cells. 
Standard 
ed I a4 7 H a4 YI x IG 
Cell. 
/ te) 5 6 7 8 9 10 MW fe i 



f 15 1 %I7 18 19 20 
( To 5 Ta 3’ a 155 
Galvanometez 
Circuit. ee , 
o Va 2 4 6 8 /0 fe’ 4 16 18 20 
Arrangement for Comparison of Clark Cells. 
The electromotive force at the ends of the heating coil in our experiments had to 
be kept balanced against twenty Clark cells. These cells form part of a battery of 
one hundred cells constructed in the year 1890, about eighty of which are still in 
good condition. <A set of twenty is mounted in two rows of ten each. The cells are 
small, but as we had no means of keeping their temperature constant, their combined 
electromotive force had, whenever required, to be determined in terms of the 
standard cell. This could be quickly done as follows :—Ten cells of the form shown 
in fig. 1 were placed side by side in the same cupboard that contained the standard 
cell (fig. 3 and cc., fig. 5). The terminals of these cells—which we shall call the 
“Cupboard Clarks ”—were permanently connected to mercury cups numbered 1, 2, 
&e. (fig. 3). A board of wood was placed in front of the Clarks and contained a 
series of cups 1’, 3’, 5’, &c., permanently connected by copper strips, and a second 
series 2’, 4’, 6’, similarly connected. The cups of the two series were placed 
respectively opposite the positive and negative poles of the Cupboard Clark cells and 
