Unit of Electrical Resistance. 169 



lead, or by a gold-silver alloy, than on reproductions by mer- 

 cury ; but until Dr. Siemens, or some other competent observer, 

 also obtains thoroughly concordant results with these materials, 

 I can place no reliance on any of these means of reproduction. 



Yet it cannot be denied that the ten so-called permanent 

 standards may all vary, and that unless some means of reproduc- 

 tion be discovered before that variation takes place, the unit may 

 be lost. It would therefore be of immense importance to be able 

 to say with certainty that the unit is equal to a certain length 

 and weight of mercury, or lead, or platinum, or any other mate- 

 rial ; only I protest that if lead, platinum, or mercury be found 

 the best material for this purpose, I will not therefore take the 

 unit length and weight or section of that material as the unit of 

 resistance. 



It is this fallacy which I have always endeavoured to expose. 

 No one in the Committee has ever underrated the value of a 

 chemical means of reproduction ; on the contrary, money has 

 been freely voted, and committees yearly appointed, having this 

 sole object. Dr. Matthiessen has been opposed to mercury, and 

 to support his views has argued that at any rate hitherto the 

 value of the mercury unit remained unfixed — a perfectly justifi- 

 able argument, supported by discrepancies in coils issued, though 

 certainly not conclusive against the possibility of using mercury 

 hereafter. But until this question is settled, the Committee have 

 thought that material standards of very different materials, which 

 remained unaltered in their equality, afforded a fair guarantee of 

 permanency. Should they not remain equal, they can fall back 

 on the length and weight of fresh mercury, lead, or gold-silver 

 alloy which have been or may be found equal to the B.A. unit. 

 Thus, according to Dr. Matthiessen, the ten standards are equal 

 to wires or prisms weighing 1 gramme per metre, and of the fol- 

 lowing lengths for the various materials : — ■ 



Lead 0-44307 metre. 



Gold-silver alloy . . 0-59952 „ 

 Mercury .... 0-076505 „ 



We have therefore, as before said, all the guarantees Dr. Mat- 

 thiessen can have for permanency, with the addition of ten mate- 

 rial standards. 



Thus, to resume, the reproduction of a standard, whatever 

 that may be, by mercury or any other metal is recognized by the 

 Committee as of great importance, both as an additional gua- 

 rantee of permanency, and in case of accidental injury; but 

 hitherto they recognize no means of reproduction by mercury or 

 otherwise as established with such authority as to justify its 

 formal adoption, and they see no reason to adopt a unit length 



