ON STANDARDS OF ELECTRICAL RESISTANCE. 133 
Several eminent practical electricians were requested to advise the Com- 
mittee as to the form of coil they considered most suitable for a material 
standard, and also to furnish a sample coil such as they could recommend. 
Sir Charles Bright informed the Committee that he was ready to comply with 
the request. The point is one of considerable importance, respecting which 
it was thought that practical men might give much valuable information. Coils 
of wire may be injured by damp, acids, oxidation, stretching and other 
mechanical alterations. They may be defective from imperfect or uncertain 
insulation ; and they may be inconveniently arranged, so that they do not 
readily take the temperature of the surrounding medium, or cannot be safely 
immersed in water- or oil-baths, as 1s frequently desirable. No definite con- 
clusion as to the form of coil to be recommended, even for copies, has been 
arrived at. 
It was resolved “ That the following gentlemen should be informed of the 
appointment of the present Committee, and should be requested to furnish 
suggestions in furtherance of its object :— 
Professor Edlund (Upsala). Professor Neumann (Konigsberg). 
Professor T, Fechner (Leipsic). Professor J. C. Poggendorff (Berlin), 
Dr. Henry (Washington). M. Pouwillet (Paris). 
Professor Jacobi (St. Petersburg). | Werner Siemens, Ph.D. (Berlin). 
Professor G. Kirchhoff (Heidelberg).| Professor W. E. Weber (Géttingen).” 
Professor C. Matteucci (Turin). 
A letter, appended to this Report, was consequently addressed to each of 
these gentlemen. Answers haye been received from Professor Kirchhoff and 
Dr. Siemens, which will be found in the Appendix. The resolution arrived 
at by the Committee to construct a material standard will entirely meet 
Professor Kirchhoft’s views. The Committee have been unable entirely to 
adopt Dr. Siemens’s suggestions; but his statements as to the accuracy with 
which a standard can be reproduced and preserved by mercury will form the 
subject of further special investigation, and the Committee will be most happy 
to take advantage of his kind offers of assistance. 
A letter was also received from Sir Charles Bright, containing an ingenious 
method of maintaining a constant tension or difference of potentials. This 
point will probably come before the Committee at a later period, when Sir 
Charles Bright’s suggestion will not be lost sight of. 
The Committee also received on the 29th of Sept., after the present Report 
had been drawn up, a letter from Dr. Esselbach, a well-known electrician, 
who had charge of the electrical tests of the Malta and Alexandria Cable 
during its submergence. In this letter Dr. Esselbach arrives at substantially 
the same conclusions as those recommended by the Committee. Thus, his 
first conclusion is “ to adopt Weber’s absolute unit substantially, and to derive 
from it, by the multiple 10", the practical unit.” This practical unit is 
precisely that recommended by your Committee. Dr. Esselbach uses the 
millimetre 
: 10 ino’ == 
multiple 10”, starting from the second ? 
where your Committee recommend 
the multiple 10’, starting from the mete : the result is the same. 
second 
Dr. Esselbach’s next conclusion is also of great practical value. He points 
out that the electro-magnetic unit of electromotive force, also multiplied by 
10”, differs extremely little from the common Daniell’s cell, and that, without 
doubt, by proper care such a cell could be constructed as would form a 
practical unit of electromotive force. This suggestion has the approval of 
