452 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE 
electromotive force of the same kind as appear in the Tables VI.," VII., VIII., 
showing the relative values of the various cells. The mean temperature at the times 
of the determinations differs so little from 15°, that the final number for that 
temperature is almost independent of the temperature coefficient. 
We may take as applicable with but little error to all the cells of this type that 
have been experimented upon 
E=l*454 B.A. volts at 15°. 
The value for the H-cells would be a little higher. (See Tables.) 
The corresponding number found by Mr. Latimer Clark was 
E= 1*457 B.A. volts, 
so that the difference between us is small, and perhaps even dependent upon variations 
in the materials or construction of the cells. 
To express our results in true volts we have only to introduce the factor expressive 
of the B.A. unit in terms of the ohm. If in accordance with our own determinations 
we take 
1 B.A. unit =*9867 ohm, 
we shall have as the value of a Clark cell at 15° 
E=1*435 volt. 
§ 38. It has been mentioned that on March 29 silver deposits were made at the same 
time as the observations of E.M.F. One object of this was to exemplify the procedure 
which will probably be in future the most convenient for the determination of E.M.F. 
when the very highest accuracy is not required. It is evident that if we assume 
a knowledge of the electro-chemical equivalent of silver, the weights obtained in a 
given time on March 29 will lead to a determination of E.M.F., independently of 
the current weighings. We propose to exhibit the method of calculation, ignoring 
altogether the use of the current-weighing apparatus, whose only effect will be that of 
a resistance of about 40 ohms. If W be the weight of silver deposited in the time t, 
w the electro-chemical equivalent, we have as the relation between W and E, 
W= 
i--V« 
pj 
On this occasion W=l*4531 grms., f=3599 seconds, It=4'0014 B.A., p=4999 B.A., 
as before. If w be assumed, the only other element required for the evaluation of E is 
