the Unit of Electrical Resistance, 327 



of this systematic and coherent method of measurement is very 

 convincingly put forward, the method proposed by Professor 

 Thomson for determining- the ~^j unit is developed, and, 

 lastly, the modus operandi of the experiments which have been 

 made, and the results obtained, are given in detail. 



The names alone of William Thomson and Clerk Maxwell 

 are a sufficient guarantee of the high scientific value of these 

 researches. Indeed they have very far surpassed in exactness 

 the previous determinations of Thomson and Weber. 



The Committee has, notwithstanding, arrived at the convic- 

 tion that even Weber's system is not adapted to a resistance- 

 unit. In the first Report the proposition is made to take, as 

 the normal measure, a material resistance-standard which shall 

 have a value as near to 10 10 Weber's units or 10 7 1^1 units as 



second 



present means enable its determination. This normal measure 

 is to remain unchanged, and, under the name "B. A. unit/' to 

 become the future measure of electrical resistance. From time 

 to time its value is to be redetermined in absolute units and co- 

 efficients of correction, published for the use of physicists in cal- 

 culating dynamical values. 



The members of the Subcommittee — consisting of Professor 

 Maxwell, Professor Matthiessen, and Mr. Jenkin, who are de- 

 puted to construct the normal standards and their copies — believe 

 that they have met the objection that the resistances of the stan- 

 dards might change spontaneously by constructing ten different 

 normals of alloys of the noble metals and of mercury, and by- 

 making the copies for distribution of au alloy of silver and 

 platinum. According to Dr. Matthiessen, these alloys are not 

 subject to spontaneous changes, whilst other metals and alloys 

 are found to show considerable variations in the course of two 

 years. 



I do not in the least undervalue the importance of Dr. Mat- 

 thiessen's experimental researches on this subject; but at the 

 same time I do not hold that his expressed opinion (that the alloys 

 of silver with either gold or platinum are electrically permanent) 

 is so absolutely proved to be a law of nature as to warrant the 

 foundation of normal resistance-standards, whose purpose is to last 

 for all time, upon the strength of it. I am rather surprised 

 that Dr. Matthiessen should have observed in German silver 

 marked changes within short periods of time, as I have always 

 found this alloy remarkably constant. But this only proves 

 that in the conducting-powers there exist still many unknown 

 factors which can only be brought to our knowledge by length- 

 ened research. » 



It is true Dr. Matthiessen brings forward an argument in 

 favour of the electric permanency of an alloy of silver with gold — 



Z2 



