H. ©. Carhart—Electromotive Force of a Damell Cell. 377 
The electromotive force of a Clark element is therefore 1°434 
volts, the exact value given by Lord Rayleigh* instead of the 
formerly accepted value of 1:457 volts. 
The mean of all the values of the electromotive force in the 
last column of the table is 1122, which is the value obtained 
by Sir William Thomson by the electrostatic method, if the 
velocity expressing the ratio between the electrostatic and elec- 
tromagnetic units be taken as 3X10”. 
In figure 2 the results of the investigation are set forth 
graphically; the ordinates represent the excess of the quantities 
in column seven of the table above 74, the value obtained with 
distilled water; while the corresponding abscissze denote per- 
centages of zinc sulphate. It will be observed that little 
variation in the value of the electromotive force occurs after 
reaching ten per cent. of zine sulphate, and that the maximum 
value appears to be at about five percent. I have some reasons 
for doubting the accuracy of the larger of the two values found 
with the five per cent. solution. If the smaller value alone is 
2. 
~ 
a eTN 
79 auee 
TT 
15 
01:8: 3. 76 15 20 25 
standard Daniell element should be so constructed as to admit 
of employing a zine sulphate solution of known concentration. 
The form proposed by Professor G. F. Barker,t in which the 
solutions are saturated appears to meet the requirements In this 
respect. 
Northwestern University, Evanston, Ill., August, 1884. 
*Nature, March 20, 1884, p. 495. In his Montreal Presidential address Lord 
Rayleigh gave the value 1°435 volts. 
+ Proc, Amer, Philos. Soc., Jan. 19, 1883, p. 654. 
