162 Mr. F, Jenkin Report on the New Unit [April 6, 



parisons, 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 tem- 

 perature was known, it was not thought necessary to specify the tempe- 

 rature 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 con- 

 ducting-power of substances is measured by comparison with that of some 

 other body, such as silver or mercury. Dr. Matthiessen has frequently 

 pointed out the discrepancies thus produced, 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 measurement, that he has been led to 

 devise the following definitions, in which none but already established 

 measures are referred to. 



The resistance of the absolute - T is such that the current generated 

 second 



in a circuit of that resistance by the electromotive force due to a straight 

 bar 1 metre long moving across a magnetic field of unit intensity* per- 

 pendicularly to the lines of force and to its own direction with a velocity of 

 1 metre per second, would, if doing no other work or equivalent of work, 

 develope in that circuit in one second of time a total amount of heat equi- 

 valent to one absolute unit of work or sufficient heat, according to Dr. 

 Joule's experiments, to heat 0*0002405 gramme of water at its maximum 

 density 1 Centigrade. 



The new standard issued is as close an approximation as could be 

 obtained by the Committee to a resistance ten million times as great as 



the absolute ,. The straight bar moving as described above in a 



second 



magnetic field of unit intensity, would require to move with a velocity of 

 ten millions of metres per second to produce an electromotive force which 

 would generate in a circuit of the resistance of the new standard the same 



* Gauss's definition. 



