782 
LORD RAYLEIGH ON THE CLARK CELL AS 
Believing that these cells are capable of affording standards of a high degree of 
precision, and that they ought to be in general use, I have gone into considerable 
detail as to the procedure which may be adopted. This may give the impression that 
the preparation is troublesome, but in reality the method that I propose is much 
simpler than those hitherto employed and thought to be necessary. To show how 
easy it is to set up these cells, I may refer to two large ones, contained in glass 
cylinders of about 4 inches diameter, and provided with wooden covers by wdiich the 
electrodes are carried. Enough common mercury was poured in to cover the bottom, 
contact being made with it by means of a platinum wire sealed in a glass tube. The 
jar was then filled to a height of about 4 inches with saturated solution of commercial 
zinc sulphate with which some mercurous sulphate had been rubbed up in a mortar. 
The zinc electrode was cut from ordinary sheet metal, and was suspended horizontally 
near the top of the liquid by a projecting tail. After the first few weeks these large 
cells have never deviated from the standard by much more than yoVo~> and have been 
found very convenient for certain purposes on account of their comparatively small 
resistance. They have also been used for preliminary comparisons with cells whose 
value was unknown, in which case there was danger of more current passing than is 
desirable to allow through delicate standards. 
§ 40. The method followed for making the recent comparisons is the same in 
substance as that described in § 28. The use of a high resistance galvanometer gave 
a greater facility of reading, a change of too'oo i n the E.M.F. under measurement 
giving a motion of the spot of light which could be seen without a telescope from 
across the room. 
The accompanying table (XIII.) gives the values of most of the older cells in 
continuation of that contained in the note to § 30 (p. 459). Cells (4), (8), (9) 
were, I think, left at Cambridge; (18) and (19) were observed at intervals during 
1885, but the E.M.F. was found to fall. When about three parts per thousand 
too low, they were removed for examination, and found to be dry. The water had 
exuded, or evaporated, through cracks in the paraffin wax. The cells of the H- 
pattern, U G , H 13 were broken in a manner to be presently explained. On the 
other hand some new cells of the H-pattern, a, l>, . . . f are included. They are those 
referred to in the previous paper as having been fitted up by Mr. Threlfall, and 
are more than a year old. 
The agreement exhibited in Table XIII. is very remarkable. In many cases 
the cells may be depended on not to vary relatively more than 2 or 3 parts in 
10,000, notwithstanding considerable changes of temperature. It is, indeed, 
doubtful whether even the whole of the small variations recorded are real. 1° C. 
influences the E.M.F. about 8 parts in 10,000, and differences of temperature of 
two or three-tenths of a degree may well have occurred, since the cells were 
variously mounted, and no particular precautions were taken beyond the avoidance 
of readings at times when the temperature of the room (immediately under the roof) 
was changing rapidly. 
