321 



intend to describe plans for attaining this object through a wide 

 range of magnitude (resistances from 10 5 to 10 13 British absolute 

 units of feet per second on Weber's invaluable system). In the 

 mean time I shall merely say that I have formed a plan which I 

 expect will prove very advantageous for low resistances, and which 

 consists in combining the standards, whichever of them are required, 

 in multiple arc (or " parallel" arcs, according to an expression some- 

 times used), so as to add their conducting powers*, instead of in 

 series, as in all arrangements of resistance coils hitherto used, by 

 which the resistances of the component standards are added. 



Part III. General Remarks on Testing by Electro-dynamic 



Balance. 



I shall conclude by remarking that the sensibility of the method 

 which has been explained, as well as of Wheatstone's balance, is 

 limited solely by the heating effect of the current used for testing. 

 To estimate the amount of this heating effect, let e and f be the 

 parts of the whole electromotive force, E, which act in the standard 

 SS', and tested conductor TT' respectively ; so that, in accordance 

 with the notation used above, we have 



SS' 



,+TT' 



SS'+R+'__ 



TT' r * ' 



/ T~l A A 



J ~ SS'+R+TT'J 



of its substance. Following Weber, I define the resistance of a bar 

 or wire -one foot long, and weighing one grain, its specific resistance. 

 It is much to be desired that the weight-measure, rather than the 

 diameter or the volume-measure, should be generally adopted for 

 accurately specifying the gauge of wires used as electric conductors. 

 With reference to either SS' or TT' (the first, for instance), let us 

 use the following notation : 



/ its length in feet ; 



w its mass per foot in grains ; 



* the specific heat of its substance ; 



a the specific resistance of its substance. 



* The reciprocal of the resistance of a " conductor" or "arc" I call its con- 

 ducting power. The conducting power of a bar or wire of any substance one 

 foot long and weighing one grain, I call the specific conductivity of its substance. 



