76 REPORTS ON THE STATE OF SCIENCE. 
of the design are in progress. It is hoped to realise the ohm in absolute 
measure to 1 part in 100,000. The experience gained in the construc- 
tion of mary of the fittings of the ampere balance will greatly facilitate 
the work. 
(b) International U: nit.—Further comparisons of some of the mercury 
standards of the National Physical Laboratory were made in October and 
November 1906. There appears to have been no change in any of the 
tubes which affects the resistance of the contained mercury columns by 
as much as 1 part in 100,000. The following table gives the observed 
differences in 1903 (the year of their construction) and in October and 
November 1906. 
Mercury Standards Observed Difference in Observed Difference in 
Compared. Int. Ohms in 1903. Int. Ohms in 1906. 
M-—P 0:00069, 0:00068, 
M-T 88, 895 
M—U 94, 95, 
M-—V = 29, - 30, 
M-X 01, 02, 
2. The Ampere. (a) Absolute Measwre—When the ampere balance 
was designed it was hoped by means of it to measure a current in absolute 
value to 1 part in 10,000, but it will be seen from the report on the 
balance that the evaluation of a current of nominal value | ampere is 
subject to an error which appears to be not greater than | part in 50,000. 
(b) International Unit of Current.—The International Conference on 
Electric Units at Charlottenburg (1905) reaffirmed the definition of the 
Jnternational ampere in terms of the deposit of silver in a silver voltameter 
or coulometer, but expressed the opinion that the information before it 
was insufficient to enable it to propose any alteration in the formerly 
accepted value for the ampere, or to lay down exact directions in respect 
to the silver voltameter. 
The Rayleigh type of voltameter has been used in a large number of 
investigations, but the researches of Rodger and Watson, Richards, Kahle, 
and others have shown that this voltameter as generally employed gives 
results which may vary as much as | part in 1,000. 
In the research at the National Physical Laboratory a reproducible 
type of voltameter was sought, but after making a large number of 
observations on various forms it was found that, subject to certain easily 
attained conditions, all the forms give identical results to 1 part in 
100,000. As the Rayleigh type is the simplest to erect and produces the 
least variation in the current strength, it is proposed that this form be 
specified. The conclusions arrived at in the research differ appreciably 
from those of most other observers, and attempts have been made expe- 
rimentally to reproduce the conditions under which they worked. In 
part we have been successful, but there are still anomalous results for 
which we can at present offer no explanation. 
It is certain, however, though the complete chemistry of the silver 
voltameter or coulometer is unknown, that a reproducible type can 
be specified, and that the International ampere can be defined in terms 
of the deposit of silver with very great accuracy, certainly to | part in 
100,000. 
The Standard Cell.—For the past five years experiments have been 
made at the National Physical Laboratory on Clark and on Weston 
cadmium cells, and two years ago a provisional specification of the 
