388 
MR. E. H. GRIFFITHS ON THE VALUE OF 
to the Cavendish Laboratory.* These cellst were (early in August) in close agree¬ 
ment with No. 31, and also with the Cavendish. They have since been compared 
with the Berlin Standard, and appear to exceed that standard by about ‘0002 volt. 
It will be noticed that the results of the comparison on September 6th are in 
much closer agreement than those of previous dates. This is partially due to the 
potentiometer, since the adjustment in this case has not to proceed by steps. It 
must be remembered also that the cells had been kept (by the regulator above 
described) at a steady temperature for a considerable time ; and the great impoi’tance 
of a constant temperature is rendered evident by the figures given in Table I., and 
illustrates the truth of Mr. Glazebrook’s remark that many discrepancies are due 
to the different “ lag ” of the cells when their temperature is changing. 
A consideration of the above facts will, we believe, bear out the conclusion that 
the mean value of our Clark cells may be regarded as exceeding that of the 
Cavendish standard by not more than '0002 volt {i.e., by about 1 in 7000). During 
our J experiments we always used a considerable number of these cells, placing 
several in parallel arc, and in no case did we use less than nine of them at any one 
time. Thus no correction for individual cells is necessary. 
A reference to Glazeerook and Skinner’s paper will show that but a short time 
elapsed between their determination of the absolute value of the Cavendish standard, 
and the comparison of our cells with that standard. The absolute value of the 
standard is given by them as 1'4342 volt we are therefore justified in assuming 
the mean value of cells No. 31 to 54, as 
1-4344 [1 + -00077 (15 - t)], 
[Note by E. H. G., added April, 1893. All the cells mentioned above practically 
maintain their relative positions at this date. I have recently taken a series of 
observations in the hope of throwing some light on the gradual approximation of the 
cells, as shown by Table I. I find that if the temperature is suddenly raised, and 
then kept quite constant, the initial differences steadily decrease with lapse of time, 
and that the cells continue to converge after the lapse of many days, and even weeks. 
The cells prepared by my method of adding some Hg 2 S 04 , to the solution before 
filtering, appear to settle into their final state more rapidly than the others.] 
Section VII. —The Measurement of Resistance. 
The coil used by us in 1888 was of platinum-iridium wire, and had a mean R of 
about 19 w. 
* We take tki.s opportunity of thanking Profe.ssor Thomson for liis kinilness in permitting ns to 
horrow many pieces of appa.ratus belonging to the Cavendish Laboratory. 
f Tliese are the cells Nos. 65, 69, 70, 77, 78, and 79, mentioned in IMessrs. Glazebrook and Skinner s 
paper. No. 65 may be regarded as equal to our No. 31. See ‘ Phil. Trans.,’ A, 1892, pp. 605, 623, and 625. 
1 ‘Phil. Trans.,’ A, 1892, p. 582. 
