1865.] of Electrical Resistance. 161 



in the use of the copies which have been issued it has been thought that 

 sufficient accuracy would be attained by the use of amalgamated mercury 

 connexions. 



In the standards themselves permanence is the one paramount quality 

 to be aimed at ; but in copies for practical use a material which changes 

 little in resistance with change of temperature is very desirable, as other- 

 wise much time is lost in waiting till coils have cooled after the passage of 

 a current ; moreover large corrections have otherwise to be employed when 

 the coils are used at various temperatures ; and these temperatures are fre- 

 quently not known with perfect accuracy. German silver, a suitable mate- 

 rial in this respect, and much used hitherto, has been found to alter in 

 resistance, in some cases, without any known cause but the lapse of time, 

 since the change has been observed where the wires were carefully protected 

 against mechanical or chemical injury. A platinum-silver alloy has been 

 preferred by the Committee to German silver for the copies which have 

 been made of the standard. These have been adjusted by Dr. Matthiessen 

 so as to be correct at some temperature not differing more than 1 from 

 15 0> 5 C. The resistance of platinum-silver changes about O031 per cent, 

 for each degree Centigrade within the limits of 5 above and below this 

 temperature ; this change is even less than that of German silver. The 

 new material seems also likely to be very permanent, as it is little affected 

 by annealing. The form of the copies is the same as that of the standard, 

 with the exception of the terminals, which are simple copper rods ending 

 in an amalgamated surface. Twenty copies have been distributed gratis, 

 and notices issued that others can be procured from the Committee for 

 2 10*. The Committee also propose to verify, at a small charge, any coils 

 made by opticians, as is done for thermometers and barometers at Kew. 



Dr. Matthiessen reports, with reference to the question of reproduction, 

 that given weights and dimensions of several pure metals might be em- 

 ployed 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 

 respectively 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 Siemens' 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 absolute resistance. The above four com- 



* If Dr. Matthiessen uses the sp. gr. of 13-596, as given by Kegnault, the difference 

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



