616 
MESSRS. R. T. GLAZEBROOK AND g. SKINNER ON THE 
was 0. The mean gives us as the difference when the two cells are at the same 
temperature the value 1, so that the change for 17° is 74‘6 — 1, and the change per 
1° C. is 4-33. 
On August 12 the E.M.F. of the Clark was found to correspond to 5736 of our 
units, and we have then as the coefficient of change of E.M.F. with temperature the 
value 4*33/5736 or ’000755. Lord Rayleigh gives as the value of the coefficient 
from about 0° to about 15° ‘000765, for another it is ‘00073, and for a third ‘00083. 
The hist three observations in Table XIV. show that for a range of temperature 
from 14°*4 to 0 ° our cell and Lord Rayleigh’s cell No. 1 have the same E.M.F. 
When the cell was put into the warm water on August 17 readings were taken of 
the E.M.F. and temperature. 
About half-an-hour after being put into the bath the E.M.F. was 45 units below 
the standard, the temperature was 31°, that of the standard 17°‘3. The readings 
remained steady at this for five minutes. 
This would correspond to an increase of only about 3‘3 units per 1° C. instead of 
4‘3, and again emphasizes the fact that the E.M.F. of the cell is not a fixed quantity 
at a given temperature unless the temperature has remained steady for some time 
previously. 
Most of our cells were compared with the standard at various temperatures 
between 9° and 18°. The observations show that all the cells except those known to 
lie unsaturated have the same coefficient, and the average value of that coefficient 
between 0° and 15° is ‘00076. Our experiments do not show whether the relation 
between E.M.F. and temperature is linear or not. 
§ 27. Portable Form of Cells. 
Cells which contain much mercury are not suitable for carrying about for testing- 
purposes, as the mercury may be thrown in contact with the zinc rod and become 
contaminated with zinc. It has been shown that a very small percentage of zinc 
dissolved in mercury causes that to behave as though it were zinc. To avoid the 
use of much mercury cells have been constructed with merely an amalgamated 
platinum wire for their negative metal. The platinum being more negative than 
mercury does not interfere with the E.M.F. of the cell. Cell No. 71, described in 
Section 20, is constructed with an amalgamated platinum wire. Since it is only 
necessary to use a metal more negative than mercury for the negative pole and to 
amalgamate its surface, we have tried the use of small antimony rods. A small rod 
of antimony is cast and boiled in mercury to amalgamate its surface. It is then 
fitted into the cell, as shown in the diagram, fig. 6. It is of importance that the 
antimony be pure, for should it contain metals more positive than mercury the 
electromotive force of the cell will be reduced by their solution in the mercury. For 
this purpose, therefore, antimony metal reduced from pure tartar emetic should be 
used. 
