60 
PHYSICS: ROSA AND VINAL 
of mercury for the ohm or the vStandard cell for the volt, is possible. 
The value of an electric current may, however, be ronveniently expressed 
for the purposes of precise measurement in two ways. Either the sil- 
ver voltameter may be used to calibrate an ampere balance as was done 
by the British Board of Trade, or the voltage of the Weston normal cell 
may be determined when a measured current passes through a known 
resistance. The latter method was adopted by the London Conference, 
but at that time information was lacking as to the proper value to assign 
to this cell, which was adopted by the conference in place of the Clark 
cell. 
In 1910 a committee consisting of representatives of the National 
Laboratories of England, France, Germany, and the United States met 
in Washington and made careful voltameter measurements to determine 
the voltage of the Weston normal cell, in the manner agreed upon at 
London in 1908. The Committee found the value to be very nearly 
1.0183 volts at 20°C. This value, recommended by the Committee 
as the international value of the Weston normal cell, has become the 
basis of measurement in general use for voltage and (with a resistance) 
for current measurements also. Although the cell has thus become 
the practical standard, the voltameter has not lost its importance, for 
it is the ultimate standard which is to be depended upon in the future 
to determine whether the standard cells preserved by the various national 
laboratories are maintaining an unchanging voltage. 
The researches on the voltameter which have been made since the 
international technical committee adjourned have been in part for the 
purpose of finding out with greater precision (to the sixth significant 
figure) how close the value for the cell as computed from the voltameter 
results and the international ohm is to the value 1.0183 volts adopted 
by the Committee. Although the difference is inappreciable for most 
purposes, it is desirable that it be determined as accurately as possible. 
It is also very necessary to have adequate specifications for the volta- 
meter that shall enable us to obtain results in the future comparable with 
those obtained now. In this work the Bureau of Standards has been 
able to take an active part. Many sources of error in the voltameter 
have been discovered and means for their elimination provided, and 
the voltameter may now be considered capable of being used to check 
the voltage of the Weston normal cell. 
During the course of the work differences of opinion have arisen among 
some of the investigators in this and other countries, but by co-opera- 
tive experiments and correspondence practically all of these differences 
