3.26 M. W. Siemens on the Question of 



But this will never be the case ; therefore Weber's unit cannot 

 be adopted for general resistance measurements — although it is 

 obviously of the last importance to determine, as exactly as pos- 

 sible, the relation between this and the unit to be chosen. 



In the establishment of a general unit of resistance, its prac- 

 tical advantages, rather than the scientific harmony of systems 

 of measurement, should be taken most prominently into con- 

 sideration. And viewed in this light — seeing that determina- 

 tions of resistances, combined with dynamic values, occur only 

 very rarely and in strictly scientific instances, while the far over- 

 whelming number of cases, on the contrary, consist of simple 

 comparisons of the resistances of bodies of different sizes, shapes, 

 and materials — a resistance-measure founded upon a corporeal 

 basis gains many reasons for being preferred to a dynamic 

 system. 



It is by such considerations that the system proposed by me is 

 recommended. In the definition of the unit, I have given the 

 metre as the measure of space, and mercury as that conductor 

 which is, without doubt, the best suited to serve as unit of con- 

 ducting-power. The reproduction of this unit is practicable 

 with sufficient exactness — indeed, when the greatest possible 

 care is observed in manipulating, probably with an almost un- 

 limited exactness.. 



Of the soundness of these grounds I have not yet found any 

 disproof based upon internal shortcomings. 



On the other hand, Dr. Matthiessen (in 1861) proposed, 

 instead of mercury, a certain alloy of gold and silver as the 

 resistance-unit — a system which any one practically acquainted 

 with the difficulties attendant on the. reproduction of a homo- 

 geneous alloy, on the forming it, when obtained, into wires 

 of perfectly equal length and section and degree of hardness, 

 without stretching or bending, and, lastly, on the soldering of 

 the ends to thick connexion-terminals without altering the 

 resistance, will scarcely at first sight be prejudiced in favour of. 



I need not go further, however, into the merits or demerits of 

 the system, as Dr. Matthiessen, at a later date, abandoned it as 

 unit in favour of the proposition made by the Committee ap- 

 pointed by the British Association in 1861 to report on the 

 most expedient unit for general adoption, of which Committee 

 he is an active member. 



The Committee has presented to the Association, at its dif- 

 ferent Meetings, four Reports, of which I have received those 

 of 1862-64. In these Reports the theory of Weber's dynamic 

 system of measurement, embracing the unit of mechanical effect 

 introduced by Professor Thomson, is explained with much clear- 

 ness, the great scientific importance of a general introduction 



