A STANDARD OF ELECTRO-MOTIVE FORCE. 
791 
may be inclined to ask whether it would not be better to prescribe a dilute solution 
for standard cells, as is conveniently done for standard Daniell’s. One advantage 
attending this construction is (as will presently appear) a lower temperature co¬ 
efficient. Again, we should be inclined to expect a more definite dependence upon 
temperature. In order to bring a saturated cell to its normal condition after warming 
(for example), it is necessary not merely that the whole of the contents should acquire 
the new temperature, but also that sufficient time should be allowed for diffusion. 
If the solution in contact with the zinc be weaker than corresponds to saturation 
at the altered temperature, the full loss of E.M.F. will not be experienced. In this 
respect the H-cells, in which the excess of salt rests upon the metals, would seem to 
have an advantage. But I cannot say that in practice I have met with the defect 
due to imperfect diffusion. Cells which have stood at 10° or 12° seem to acquire 
their new values after an hour or two’s immersion in ice. The argument weighs, 
however, in favour of smcdl cells, through which diffusion of temperature and matter 
can take place quickly. Such experience as I have had of cells prepared with dilute 
solutions, would not lead me to prefer them, even were there no difficulty in, or 
necessity for, a standardising. In the case of clear solutions, such as are used for 
Daniell’s, the specific gravity is a convenient test of strength ; but I do not see how 
a standard unsaturated paste could be accurately prepared without a good deal of 
trouble. Another objection to dilute solutions is the progressive alteration of E.M.F., 
due to evaporation, which must take place whenever the sealing is at all imperfect.* 
In truth, there is no real difficulty in avoiding both under and over-saturation, if 
the experimenter will bear in mind the known properties of the materials with which 
he is dealing. The grosser errors, arising from the first cause, can only occur as 
the result of carelessness. As to the latter, it may be that supersaturation has 
sometimes entered as a consequence of excessive precautions against the admission 
of air. It cannot occur in the presence of the minutest fragment of the normal 
hydrate. Opinions may perhaps differ upon this point, but I am myself disposed 
to condemn the use of heat in charging the cells. If hot paste be brought into 
contact with hot mercury, and then closed hermetically, there must be some risk of 
supersaturation. 
§ 52. The next question which I propose to consider is that of the temperature- 
coefficients of Clark cells. My observations on cell No. 1 at Cambridge, § 36, gave for 
the proportional fall of E.M.F. per degree Centigrade in the neighbourhood of 15° the 
number '00082, so that at t° C. we might take 
E=l'435{l-'00082(£-15)}. 
This number is in agreement with that found by Helmholtz for. saturated cells, but 
it differs seriously from the number ('00041) given by Alder Wright! also for 
* Alder Wright. Loc. cit., p. 33. 
f Phil. Mag., July, 1883, p. 36. 
5 I 2 
