110 REPORTS ON THE STATE OF SCIENCE.—1912. 
is concerned an accuracy within at least five parts in 100,000 can still be 
guaranteed. This conclusion is greatly strengthened by the results 
which were communicated to the Association last year by Dr. Dorsey of 
the Bureau of Standards, Washington. At that institution Drs. Rosa 
and Dorsey have made experiments with a new current balance, the. 
coils of which are arranged in a manner similar to those used by Jouie 
and by Lord Rayleigh. They obtained results for the electromotive 
force of the Weston normal cell which agree with those obtained at the 
National Physical Laboratory within four parts in 100,000. Whether 
this represents a real difference in the results given by the two balances, 
or is an actual difference in the E.m.F.’s of the reference cells used, has 
not yet been decided. 
With regard to the absolute measurement of current elsewhere, .a 
current weigher has been built at the Laboratoire Central d’Electricité, 
Paris, and at the Reichsanstalt further measurements are to be made in 
the near future. It will be seen therefore that the absolute measure- 
ment of current is on a very satisfactory basis. At the National 
Physical Laboratory no efforts will be spared to maintain the Ayrton- 
Jones balance in good condition and to obtain results equal in pre- 
cision to those obtained at the present time. 
Turning now to the absolute measurement of resistance. For many 
years no measurements of this quantity have been carried out, but at 
the present time the Lorenz apparatus at the National Physical Labora- 
tory and other apparatus now being constructed at Berlin and Washing- 
ton will place measurements of resistance in a position equally satis- 
factory with those of current. The Lorenz apparatus is now being 
employed for the measurement of resistance, and it is believed that the 
probable error will not exceed two parts in 100,000. This satisfactory 
state of affairs is largely due to the design and size of the apparatus and 
the ease with which the dimensions of the coils can be measured. Many 
years ago Lord Rayleigh showed that it was not necessary to measure 
accurately the diameter of the coils of a Joule balance; the ratio of the 
diameters was sufficient, and this ratio could at any time be obtained 
by measuring the ratio of two currents. In consequence, with a Joule 
balance an observer is not handicapped in his measurements by the 
results of linear observations which may have been made many years 
previously and which may be incorrect owing to secular change. With 
the Lorenz apparatus independence of previous linear measurements 
has been secured by winding the coils with bare copper wire and leaving 
them in this condition. This enables linear mgasurements to be made 
at any time with ease and with precision. 
Referring now to material standards, it is most gratifying to record 
that measurements of resistance, of current, and of electromotive force 
are now made on the same basis in practically all civilised countries. 
This satisfactory state has been achieved within the past four years and 
is a direct result of the labours of the London Conference of 1908; in 
which this Committee was so largely interested. tides 
As is well known, the International standard of resistance is that 
of a specified column of mercury, and that of current depends on 
measurements with the silver voltameter. The measurement of electro- 
