ON STANDARDS OF ELECTRICAL RESISTANCE. 303 



Committee have been. much assisted by the experiments on permanency made 

 by Dr. Matthiessen. 



Silver and copper were found to alter in their resistance simvly by age. 

 German silver was also found to alter in some cases. 



These materials had therefore to be rejected. Gold appears constant, but 

 owing to its low specific resistance a considerable length would have been 

 required, unless a wire had been adopted of very small diameter. This was 

 not thought desirable, for several reasons ; any slight decay or injury in the 

 surface of a small wire woidd cause much greater alteration in the resistance 

 than the same injury to a large wire. A small wire would be more liable to 

 mechanical injury, and would be much more rapidly heated by the passage 

 of currents. The Committee having rejected small wires for these reasons, 

 thought it unnecessary to incur the expense of a large and thick gold wire. 

 The great change of resistance caused by a change of temperature furnished 

 another reason for rejecting gold and other pure metals. One pair of 

 standards was, however, made of platinum, which appeared the most suitable 

 of all the pure metals. Platinum and the three alloys named appear all to 

 be very constant, that is to say, their resistance is not altered by age, or 

 even by being subjected to considerable heat and recooled. 



These materials also possess considerable mechanical strength ; they are 

 not easily injured by chemical action, they have considerable specific resist- 

 ance, and that resistance, in the case of the three alloys, changes little with 

 a change of temperature. 



It is of course impossible to say with certainty that their resistance will net 

 vary with time, but it is most unlikely that the resistance of all will vary in 

 the same ratio. If, therefore, as is hoped, the eight coils made of such 

 different materials retain their relative values, some confidence may be felt 

 in the permanence of the unit. 



Some additional security is given by the power of reproducing the unit, if lost, 

 by chemical means, or by fresh experiments on absolute electro-magnetic mea- 

 sure, although neither of these means at present appear to give such pei feet 

 accuracy as would be secured by the permanency of a material standard. Fresh 

 absolute experiments of the kind described in previous Reports would hardly re- 

 produce the same value much within one part in a thousand, and Dr. Mattbies- 

 sen,as appears from last year's Report, is not veiy sanguine of obtaining a better 

 result than tins by chemical means. Thus a difference exists in Dr. Siemens' s 

 and Dr. Matthiessen's reproduction of a unit by means of merctuy, as 

 pointed out in last year's Report. It is of course probable that differences of 

 this kind will in time disappear, and Mr. Siemens fairly points out that the 

 discrepancy mentioned in last year's Report, between coils made from a very 

 old and those made from a new determination of the mercury unit, affords 

 no criterion of the accuracy w 7 ith which mercury can now be used as a means 

 of reproduction. Dr. Siemens was the first person who produced numerous 

 sets of coils accurately adjusted ; and although unable to recommend the 

 adoption of his unit of resistance, the Committee once more take an oppor- 

 tunity of expressing their sense of the high value of Dr. Siemens's researches 

 on the reproduction of units by means of mercury. Dr. Siemens is confident 

 that a unit can be and has been reproduced by means of mercury with an 

 accuracy of 0-05 per cent. ; but meanwhile, the chief security for the per- 

 manency of the unit consists in the preservation of standards constructed in 

 various ways and of various materials. 



The mercury tubes furnish an additional security. A molecular change 

 may occur in the wires, that is to say, they may become of harder or softer 



