1895.] Mr. J. Viriamu Jones on Electrical Resistance, 601 



WEEKLY EVENING MEETING, 



Friday, May 24, 1895. 



William Crookes, Esq. F.R.S. Vice-President, in the Chair. 



J. Viriamu Jones, Esq. M.A. F.R.S. M.B.I. 



The Absolute Measurement of Electrical Resistance. 



If we take a conductor, such as this piece of copper wire, and make 

 it part of a conducting circuit in which an electric current is flowing, 

 we find that the electromotive-force between its extremities is pro- 

 portional to the strength of the current passing through it, so long 

 as it remains in the same physical condition. If either the electro- 

 motive-force or the current strength varies, the other of these 

 quantities varies in like proportion. Their ratio is constant, i. e. it 

 has a value independent of them, and depending only on the dimen- 

 sions of the conductor, the nature of the material of which it is made, 

 the state of aggregation of its parts, and its temperature. 



This ratio is called the electrical resistance of the conductor, and 

 so defined it corresponds to a real physical quantity — it is a physical 

 property of the conductor. 



Probably no physical measurement can be made with greater 

 accuracy than the comparison of electrical resistances. Such com- 

 parisons are daily made in many parts of the world, and it is clearly 

 desirable, in order that the results obtained by one man may have 

 meaning for others, that all should make their reckoning in terms of 

 the same unit. Accordingly, the scientific world has given great 

 attention to the definition of such a unit, and much international 

 negotiation has taken place with a view of securing world-wide 

 agreement on this point. 



The most obvious method of procedure is to fix upon some con- 

 venient conductor as a standard, and to call its electrical resistance 

 the unit of electrical resistance, other electrical resistances being 

 then expressed as so many times or such and such a fraction of the 

 resistance of this standard conductor. 



The disadvantage of this method of defining the unit is that the 

 resistance of such a standard may change from time to time through 

 alterations of its physical condition. The most notable change is 

 consequent on change of temperature ; this may, however, be allowed 

 for by defining the unit as the electrical resistance of the standard 

 conductor when it is at a specified temperature. But the other 

 changes of physical condition, changes in the state of aggregation of 

 the parts of the conductor, effects of strain, alterations of molecular 

 structure, &c, are more serious, because they are unknown, and we 

 cannot in our definition provide against unknown possible changes. 



