ON STANDARDS OF ELECTRICAL RESISTANCE. 347 



science, and will always remain insignificant for any practical applications. 

 For these applications it is chiefly important that every copy of the standard, 

 whatever that may be, should be accurately made — a condition which is quite 

 unaffected by the greater or less discrepancy between the standard and true 

 absolute measurement. 



The reproduction of the standard can perhaps be more easily effected, if 

 ever it be necessary, by a given weight of metal or alloy than by a fresh 

 absolute determination. 



Meanwhile practical standards of resistance are ui'gently required, and the 

 Committee are pressed to come to a decision. Defective systems are daily 

 taking fii-mer root, and the measurement of currents, quantity, capacity, and. 

 electromotive force caU ui'gently for the attention of youi* Committee. 



Under these circumstances they have decided to rest content with the 

 results of the experiments now completed, and to commence at once the 

 construction of standard coils. 



The details of the expeiiments on absolute resistance are given in Ap- 

 pendix A. 



It may be useful here to mention that the new unit wUl be roughly equal 

 to 0'0736 times Dr. Matthiessen's mUe of copper wire, and more exactly 1'0456 

 times Siemens's unit, according to standards which have kindly been sent by 

 Dr. Siemens to several members of the Committee and others*. 



Th€ questions of chief importance, after the magnitude of the standard has 

 been chosen and determined, concern the choice of a suitable form and mate- 

 rial for the actual construction of the standard, and in this choice the per- 

 manence of the standard is above all essential. 



Dr. Matthiessen has for two years been endeavouring, at the request of 

 the Committee, to discover whether the electrical resistance of various metals, 

 under various conditions, can be considered as constant, or can be proved to 

 alter. His Eeport for the present year is given in Appendix B, and will be 

 foimd to confirm, in a great measure, the conclusions arrived at in his Eeport 

 for 1863. 



No variation has been observed by him in the electrical resistance of an- 

 nealed wires of silver, copper, gold, platinum, nor in the hard-di'awn wires 

 of gold, platinum, or of the gold-silver alloy. But a change has been observed 

 in the hard-drawn wires of silver and copper — a change most rapid in the 

 first year, but very sensible in the second year ; a somewhat capricious change 

 has also been observed in certain annealed German-sUver wires, while others 

 have been proved constant. This result has been independently observed by 

 other members of the Committee. In the hard-di-awn wires of silver and 

 copper the direction of the change has been such as to bring the resistance 

 of hard-drawn wires more nearly to resemble that of annealed wires, dimi- 

 nishing the resistance ; in other words, it is such a change as would be pro- 

 duced by partial annealing. 



From these experiments it is clearly itndesirable that silver or copper 

 should be used for standards even in their annealed state ; and the change in 

 these metals further indicates that for standards of other metals the partially 

 annealed is preferable to the hard-drawn condition. 



The experiments on these points must be continued for many years before 

 much reliance can be placed on the results ; and meanwhile equal standards 



* Twenty-five units are w-itliin one per cent, equal to tiie mile of No. IG copper wire in 

 use by the Electric and International Company. Mr. Varley has promised that for the 

 future exact equality shall be aimed at. 



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