j 
Hl. 8. Carhart—Electromotive Force of a Daniell Cell. 375 
of the resistance inserted between C and E and the strength of 
the current in the main circuit equals the difference of potential 
between the two points, or the electromotive force of A. 
The current was measured by a silver voltameter (S) with 
Bae nitrate of silver The resistance employed at R was in 
lemens units. The deposition of silver was continued for ten 
minutes; the eup was then washed with great care, dried in a 
hot air chamber, and weighed after cooling, fractions of milli- 
grams being obtained by observing the swing of the pointer. 
The Daniell cell consisted of a U tube of the form employed 
by Kohlrausch in measuring the resistance of electrolytes,* 
the bend of the tube being much smaller than the two branches. 
The lower portion was first filled with a saturated solution of 
pure zine sulphate; saturated copper sulphate was then added 
to one branch and a percentage solution of zinc sulphate to 
the other. The sulphates were so added that the surface of 
| 
about two meters. After a little experience a balance was 
readily obtained by the greater or less immersion of the silver 
plate in the nitrate of silver solution of the voltameter; and it 
Millimeter force divisions. The electromotive of A was thus 
Measured with minimum liability to polarization and by a 
method entirely independent of the internal resistance of the cell. 
he only. resistance required is that between C and H, and this 
Can easily be measured by means of a rheostat. 
he temperature of the rheostat and of the cell was observed 
at every trial. The extreme limits of variation in the tempe- 
Tature of the cell was 3°-2 C. 
e table exhibits the results; but the final reduction, as 
Shown in the last column, was not made till after the recent 
report of the Paris commission on the legal ohm, and the pub- 
lication of the results of Lord Rayleigh’s experiments with the 
Silver voltameter.t The values of the electromotive force, given 
in the column next to the last in arbitrary units, were reduced 
to volts as follows :— 
* Wiedemann’s Elektricitat, p. 590. 
+ Nature, March. 20, 1884, p. 495. 
