416 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
we could have safely done if the length of time during which the current passed had 
to be measured with great accuracy. 
As regards the dimensions of the apparatus, there is not very much choice. 
The voltameter limits the current intensity to about one ampére. The calorimeter 
must contain at least one litre of water, otherwise the corrections for cooling become 
too large ; on the other hand difficulties of stirring appear when the mass of water is 
too great ; the calorimeter in our experiments contained about 1500 grams. of water. 
In order to measure the rise in temperature of the water with sufficient accuracy, 
that rise should not be less than 2°; and it is easily calculated that if the experiment 
is to be completed in about ten minutes, the electromotive foree—with the current 
and the quantity of water fixed upon—must be about thirty volts. We used Clark 
cells as our standard of electromotive force, and a battery of twenty cells was found 
sufficient for our purpose. Having fixed on the current and electromotive force, the 
resistance of the wire can be determined. 
The accuracy which can be obtained altogether depends on the attention given to 
small details, and we therefore proceed to describe separately the various parts of the 
apparatus used. 
The Clark Cells. 
As standard of electromotive force we used Clark cells. It was originally intended 
to keep one of these cells at a constant temperature of about 25° C. in a thermostat. 
This cell (fig. 1), which we shall call the C.T.C. (constant temperature Clark), was 
constructed according to a pattern which has proved very useful when it is desirable 
to have a cell which can stand without damage a moderate current for a short time. 
A small Woutrr’s bottle is taken in which the three openings have stoppers made of 
glass ground into the necks. The stopper fitting into the central opening has a 
platinum wire fused into it, and the zinc rod is attached to this wire. Of the two 
other openings, one carries a bent tube with a mercury trap, and the other a glass 
tube which passes to the bottom of the bottle. A platinum wire passes through this 
tube and dips into the mercury. The mercury trap is useful in the early history of 
the cell, as it is difficult to avoid generation of some gas at first, and the additional 
pressure thus produced often damages the joints of the cell. After a time the mercury 
may be removed, and the tube sealed off. The cells were set up with the usual 
precautions, and have now worked for some years satisfactorily. There is, of course, 
the usual difficulty of ascertaining the temperature ; thermometers have been inserted 
into some of them, but even then, though they have proved of great utility, they are 
not suitable as standards when great accuracy is required, unless placed in an 
enclosure of constant temperature. 
The comparison of our Clark cells was always made by opposing them and measuring 
the difference of their electromotive forces. For this purpose the current from a 
Leclanché cell (L, fig. 5, p. 427) was passed through a fixed resistance (R) of 10,000 
