Unit of Electrical Resistance, 165 



So far I have argued the matter simply on the ground of defi- 

 nition. Dr. Siemens says that "the resistance-unit must con- 

 sist of a definition, or be an absolute measure, which can be at 

 any time and in any place reconstructed." I think I am right in 

 understanding these words to mean that Dr. Siemens prefers a 

 mere definition to any material standard whatever, even his own ; 

 and I am borne out in this view by his remark, that future and 

 more complete determinations of his unit would become neces- 

 sary as the exactness of physical measurements progresses ; 

 though he anticipates that no practical inconvenience could result 

 from these corrections, as they would be very small. The Com- 

 mittee were of a diametrically opposite opinion : they thought 

 that the one essential quality of a standard was its invariability, 

 that no possible inconvenience would be equal to the continual 

 variation of the standard, making, as it were, the 12 inches of 

 today the 13 inches of tomorrow. No doubt we all believe that 

 the corrections would not be so great or so frequent as this ; but 

 what man of science could recommend a periodical tampering 

 with the lengths of the standard metre to bring it more nearly 

 in accordance with some abstract definition ? Such changes 

 would on each occasion render almost useless an inconceivable 

 mass of scientific labour. For instance, all Dr. Matthiessen's 

 specific resistances of metals, recorded in Siemens' s units, would, 

 since their first issue, have required two or three revisions, and 

 would even now be far from fixed. If, therefore, we can make a 

 permanent unit, I think any permanent unit will be better than 

 any definition ; but if a definition alone is to be adhered to, the 

 definition adopted by the Committee is much preferable to that 

 of Dr. Siemens. 



Dr. Siemens acknowledges with perfect candour the scientific 

 importance of the determination of Weber's dynamic unit, but 

 thinks that, as the determination of resistances combined with 

 dynamic values rarely occurs, its general use is not of much 

 practical importance. But although the combination of resist- 

 ances with dynamic values rarely occurs in practice, the combina- 

 tion of resistance with the measurement of quantity, capacity, and 

 currents is of daily occurrence. The unit proposed by Dr. Siemens 

 stood alone and unconnected with any of those measurements. 

 The Committee have adopted a coherent system, in which the 

 current will be equal to the electromotive force divided by the 

 resistance, the quantity to the quantity conveyed by the unit of 

 current in the second, and the capacity to the capacity which, 

 when electrified by the unit electromotive force, will contain the 

 real quantity. Standards for the determination or measurement 

 of all these magnitudes will be issued by the Committee. Dr. 

 Siemens himself very fairly admits that in the British Associa- 



