792 
LORD RAYLEIGH OR THE CLARK CELL AS 
saturated cells. These discrepancies have naturally led to the conclusion that the 
temperature-variations of Clark cells are uncertain, and Dr. Fleming has insisted upon 
the advantage in this respect possessed by the Dantell.* If indeed it were a matter 
of chance whether the temperature-coefficient of a Clark were '0008, or '0004, the 
utility of these cells as standards for delicate work would be seriously impaired. A 
glance, however, at Table XHI.t will show that such uncertainty need not exist. The 
results of March 2 and June 4 correspond to a difference of 19° F., or about 11° C., so 
that if one cell had the coefficient ‘0008, and another the coefficient '0004, the change 
of temperature would separate them to the extent of 44 parts in 10,000, whereas the 
greatest change observed (perhaps not due to this cause at all) is but 5 parts. Many 
observations on recent cells, made in ordinary course, point in the same direction. 
In one or two cases there has been an apparent rise at temperatures above 65° F., 
indicating a drop in the temperature-coefficient relatively to that of No. 1. I have 
attributed this to an insufficient excess of undissolved salt in cells prepared when the 
weather was cold, the result of course being a failure of saturation at high temperatures, 
attended (as will presently appear) by a fall of temperature-coefficient. 
Being desirous of clearing up, as far as possible, any questions connected with the 
practical use of these cells as standards, I determined to supplement the former 
observations with special experiments at somewhat extreme temperatures, which 
should include as great a variety of constructions as possible. Most of the cells were 
so mounted that they could not well be tried at temperatures differing from that of 
the surrounding air, and I had to content myself with varying the temperature of 
the room by opening windows and burning gas. Care was taken that no great 
variation occurred within two or three hours of the comparisons. Under these 
circumstances tests were made of (l), (10) (Table XIII.) ; T 2 (one of the abnormal cells 
of Mr. M. Evans’), T 3 (a normal cell of the same batch); W 31 , W 59 , W 63 —three cells 
by Dr. Alder Wright ; M 183 , a cell sold by Messrs. Clark and Muirhead, —at tem¬ 
peratures from 47° F. to 69° F. These cells were all supposed to be saturated. There 
were also two prepared purposely with diluted paste—(39), (40). Besides these, two 
saturated cells of my own construction mounted in test tubes, which could be 
immersed in water or ice, were tested from 32° F. to 67^° F. 
In order to obtain an absolute result we must have command of a standard 
independent of temperature-variations. At Cambridge I employed for this pm'pose 
a cell kept constantly in ice. The present observations were rather protracted, and I 
preferred to rely upon two cells (35), (38), mounted in test tubes and imbedded with 
a thermometer in a mass of sand, itself situated in an underground recess. The 
variations of temperature were here very small and readily determined, so that there 
* On the “ Use of Daniell’s Cell as a Standard of Electro-motive Force,” Phil. Mag., Aug., 1855. 
It appears, however, to he, by a slip of the pen, that the coefficient for the Daniell is represented as 
only m of that of the Clark. The numbers given lead to the ratio -§•. 
t I must apologise for the Fahrenheit degrees. 
