ox STANDARDS OF ELECTRICAL RESISTANCE. 349 



experiments. The discrepancy -vrill be best explained by the following Table, 

 giving the value of a cohmrn of mercury at 0° C. one metre long, and having 

 a cross section equal to one square millimetre, according to various experi- 

 ments, and with the specific gravity used respectively by Dr. Siemens and Dr. 

 Matthiessen. 



Definition. J^f^'^^ ii 



B. A. units. 



1. Mercury unit according to Siemens's standard issued in 



1864. Sp. gr. mercury assumed at 13-557 0-9564 



2. Mercury unit according to Siemens's experiments made for 



1864 standard, but assuming sp. gr. mercury at 13-595*. 0*9534 



3. Mercury unit according to Dr. Matthiessen's experiments. 



Sp. gr. mercury assumed at 13-557 0-9646 



4. Mercury unit according to Dr. Mathiessen's experiments, 



Sp. gr. mercury assumed at 13-595 09619 



5. Mercury unit according to one set of coils exhibited in 1862 



by Dr. Siemens (Berlin) 0-9625 



G. Mercury unit according to a second set of coils exhibited in 



1862 by Dr. Siemens (London) 0-9742 



Dr. Matthiessen considers 'No. 4 the tme value, while Dr. Siemens sup- 

 ports No. 1. The Committee do^ not desii-e to express any opinion on this 

 subject, but only to draw attention to the great discrepancies which follow 

 the apparently simple definition of the mercury unit (first proposed by Marie 

 Davy). Even now it cannot be said that a trustworthy standard, answering 

 to the definition, exists. 



The Committee have little to report concerning the standard instruments 

 for the measurement of currents, quantity, capacity, or electromotive force. 

 The drawings for a standard galvanometer and electro-dynamometer have 

 been begun. An electro-djTiamometer, suitable for general use, has been 

 constructed by Professor W. Thomson, and experiments are being made Avith it. 



Professor Thomson has also had some fine apparatus made for the measure- 

 ment of electrostatic phenomena and their comparison with electromagnetic 

 measurements ; but it will be best to describe the instruments when the ex- 

 periments have been completed. 



Dr. Joule has made some preliminary experiments with the view to rede- 

 termine the mechanical equivalent of the unit of heat by electrical means. 



Thus, although the Committee have not accomplished all that they hoped, 

 they feel that such progress is being made as will justify their reappointment. 



They have received assurances tliat the British Association system of units 

 will be readily adopted in this kingdom, in India, Australia, and Germany. 

 They believe that it will be accepted in Ameiica and in many other parts of 

 the world. 



From France no response has yet been obtained. 



The Committee wish to express their sincere regret at the death of one of 

 their members, Dr. Esselbach. He had made valuable experiments on the 

 electromotive force of various chemical combinations, and had promised to 

 communicate them to the Committee ; but theu- record is now probably lost. 



Before concluding, the Committee have to thank Mr. Charles Hockin for 

 the efficient assistance he has afforded, both in the determination of the 

 resistance unit and in Dr. Matthiessen's researches. 



* 



This is the mean of the values given by Kopp, Eegnault, and Balfour Stewart. The 

 discrepancy between the two values is far greater than could be due to any confusion as 

 to the reference of the specific gravity to water at 0° and at maximum density. 



