618 
MESSRS. R. T. GLAZEBROOK AND S. SKINNER ON THE 
§ 28. Yon Helmholtz Standards. 
In the ‘Sitzber. tier Akatl. der Wiss., Berlin,’ 1882, p. 26, yon Helmholtz 
has described an electromotive force standard consisting of mercury, mercurous 
chloride, zinc chloride, and zinc. Similar cells have been prepared and used by 
Ostwald (‘ Zeitschrift fur Physikalische Chemie,’ vol. 1.) We have prepared a set 
of six cells like these, and have tested them at intervals. They are put up in small 
test tubes, with platinum wires sealed through the bases. The solution of zinc 
chloride was prepared from solid sticks of zinc chloride, and therefore probably con¬ 
tains some oxychloride. It had a specific gravity 1*445 at about 15°. Since the 
solution alters in strength if a current be taken from these cells, it is necessary to be 
very careful that they do not become short-circuited. These cells had an E.M.F. of 
about '98 volt; if the solution be taken slightly more dilute they may be adjusted to 
be exactly 1 volt at 15°. Ostwald has attempted to do this, and states that it is 
secured by using a solution of specific gravity 1 *41. 
Three series of observations were raa.de of these cells, and the following table gives 
the results in terms of the E.M.F. of a Clark as unity at the temperatures mentioned. 
Table XYI. 
Date .... 
February 17. 
March 16. 
August 22. 
Temperature 
16 
13 
17-4 
Number. 
101 
6890 
6879 
6892 
102 
6891 
6882 
6894 
103 
6892 
6882 
6895 
104 
6890 
6879 
6892 
105 
6891 
6882 
6892 
106 
6891 
6882 
6894 
Mean .... 
0891 
6881 
6893 
Value in volts . 
•9876 
•9881 
•9870 
The last line gives the value in volts, assuming the E.M.F. of a Clark at 15° C. to 
be 1‘4242 volt. 
The absolute values at the earlier dates are somewhat uncertain because of the 
uncertainty attaching to the actual value of the Clark used as standard. In this case 
the values, on February 17, were referred to Callend Ait’s cells, but it is clear that the 
cells cannot have changed much, and that they retain unaltered their relative 
values. 
