ON STANDARDS OF ELECTRICAL RESISTANCE. 157 
Professor Edlund (Upsala). Professor Neumann (Kénigsberg). 
Professor Th. Fechner (Leipzig). Professor J. C. Poggendorff (Berlin). 
Dr. Henry (Washington). M. Pouillet (Paris). 
Professor Jacobi (St. Petersburg). Werner Siemens, Ph.D. (Berlin). 
Professor G. Kirchhoff (Heidelberg). | Professor W. G. Weber (Gottingen).” 
Professor C. Matteucci (Turin), 
I have, in consequence, the honour of addressing you the present letter. 
The Resolutions passed at the two meetings are enclosed, and from them 
you will gather the general scope of the Committee’s inquiry. I add some 
further explanation as to the object and intentions of the Committee. 
Great inconvenience has been felt from the absence of any generally adopted 
unit for the measurement of electrical resistance, and it was thought that the 
influence of the British Association might be successfully exerted to procure 
the adoption of acommon standard. The present time was thought especially 
favourable, since, although the methods of observation have been brought to 
great perfection, no local units have as yet taken very deep root. 
The units which up to the present time have been considered by the 
Committee may be classed under three heads :— 
Ist. A given length and weight or section of wire made of some pure 
metal, and observed at a given temperature, as originally proposed by 
Professors Wheatstone, Jacobi, and others. 
2nd. Units based on Weber’s and Gauss’s system of absolute measure- 
ment. 
3rd. A given length and section of pure mercury at a given temperature. 
Whatever basis is adopted for the unit, it is proposed that the unit adopted 
shall be represented by one particular standard, constructed of very permanent 
materials, laid up in a national repository ; and it has been proposed to use 
Dr. A. Matthiessen’s gold-and-silver alloy for this purpose. The arguments 
which have been used for and against these systems are as follows :— 
In favour of the use of a wire of some pure metal it is said— 
That the plan is the simplest possible, and admits of independent observers 
forming their own standard. 
Against the plan it is said— 
1st. That even when pure, two apparently similar wires do not resist 
equally unless their temper or molecular condition be the same—a condition 
which cannot practically be ensured. 
2nd. That there is reason to believe that the resistance of a given wire is 
not constant even at a constant temperature. 
3rd. That the resistance of all pure metals varies very rapidly with the 
temperature. 
4th. That great difficulty is found in obtaining any metal pure, and that 
the attempt of most persons to reproduce the unit for their own use would be 
attended with incorrect results. This is evidenced by the different relative 
results as to the resistance of pure metals published by different observers. 
In favour of Weber’s units it is urged— 
Ist. That their use will ensure the adoption of a complete system of corre- 
sponding standards for electrical currents, quantities, and tension or difference 
of potential. 
2nd. That their use is essential in the dynamic treatment of any problem 
connected with electricity ; for instance, in determining the heat generated, 
the force exerted, the work done, and the chemical action required or pro- 
duced under any given circumstances. 
