484 Royal Society: — 



for £2 10s. The Committee also propose to verify, at a small charge, 

 any coils made by opticians, as is done for thermometers and barome- 

 ters at Kew. 



Dr. Matthiessen reports, with reference to the question of repro- 

 duction, that given weights and dimensions of several pure metals 

 might be employed for this purpose if absolute care were taken. 

 The reproduction, in this manner, of the mercury unit, as defined by 

 Dr. Siemens, differs from the standards issued by him in 1864 about 

 8*2 per thousand if the same specific gravity of mercury be used for 

 both observations*. Each observer uses for his final value the mean 

 of several extremely accordant results. It is therefore to be hoped 

 that the standard will never have to be reproduced by this or any 

 similar method. On the other hand, four distinct observers; with four 

 different apparatus, using four different pairs of standards issued re- 

 spectively by Dr. Siemens and the Committee, give the B. A. unit 

 as respectively equal to 1*0456, 1*0455, 1*0456, and 1*0457 of Sie- 

 mens' s 1864 unit. It is certain that two resistances can be compared 

 with an accuracy of one part in one hundred thousand — an accuracy 

 wholly unattainable in any reproduction by weights and measures 

 of a given body, or by fresh reference to experiments on the abso- 

 lute resistance. The above four comparisons, two of which were 

 made by practical engineers, show how far the present practice and 

 requirements differ from those of twenty and even ten years ago, 

 when, although the change of resistance due to change of tempera- 

 ture was known, it was not thought necessary to specify the tem- 

 perature at which the copper or silver standard used was correct. 

 The difficulty of reproducing a standard by simple reference to a 

 pure metal, further shows the unsatisfactory nature of that system 

 in which the conducting-power of substances is measured by compa- 

 rison with that of some other body, such as silver or mercury. Dr. 

 Matthiessen has frequently pointed out the discrepancies thus pro- 

 duced, although he has himself followed the same system pending 

 the final selection of a unit of resistance. It is hoped that for the 

 future this quality of materials will always be expressed as a specific 

 resistance or specific conducting-power referred to the unit of mass 

 or the unit of volume, and measured in terms of the standard unit 

 resistance, that the words conducting-power will invariably be used 

 to signify the reciprocal of resistance, and that the vague terms good 

 and bad conductor or insulator will be replaced, in all writings aiming 

 at scientific accuracy, by those exact measurements which can now be 

 made with far greater ease than equally accurate measurements of 

 length. 



There is every reason to believe that the new standard will be 

 gladly accepted throughout Great Britain and the colonies. Indeed 

 the only obstacle to its introduction arises from the difficulty of 

 explaining to inquirers what the unit is. The writer has been so 

 much perplexed by this simple question, finding himself unable to 

 answer it without entering at large on the subject of electrical mea- 



* If Dr. Matthiessen uses the sp. gi\ of 13*596, as given by Kegnault, the 

 difference from Dr. Siemens's standard is 5 per thousand 



