Siemens Mercury Unit of Electrical Resistance. 295 



So far as space allows, I will communicate the observations 

 in such a form that the calculation can be checked, and that it 

 may be possible to form an opinion as to the degree of accuracy 

 that has been reached. Independently of the consideration that, 

 in the case of any fundamental and difficult measurement which 

 lays claim to precision, the fullest detail is desirable in the pub- 

 lication of the numbers obtained, there are in the present case 

 two additional reasons for this course. In the first place, the 

 observations relate to a perfectly new and important principle 

 of galvanometry established by Weber, in connexion with which 

 the methods of observation with a swinging needle, likewise 

 introduced by him but not widely adopted as yet, have been 

 employed. In the second place, my results render probable an 

 error of about 2 per cent, in the unit of resistance adopted by the 

 British Association; and, in view of the authority attaching to the 

 Committee on Electrical Standards, which was appointed by so 

 eminent a body and composed of the first scientific names, and 

 which moreover disposed of considerable external means, there 

 seems need for the greatest caution, but also in the end for more 

 distinct emphasis. A series of objections will be found pointed 

 out further on against the determination of the British Asso- 

 ciation. 



I. On the various Methods for the Absolute Measurement of 

 Resistance. 



The determination of the absolute resistance of a conductor 

 by magnetic means always leads to the problem of measuring 

 the current which a known electromotive force produces in the 

 conductor, the unit current being taken as that which exerts 

 externally the unit of magnetic action, and the unit of electro- 

 motive force being that which is developed when a conductor is 

 moved against magnetic forces under certain normal conditions*. 

 We owe to Weber four methods for making such determinations 

 of resistance, the principles of which may be briefly stated as 

 follows : — 



In the firstf, the electromotive force induced by the earth's 

 magnetism in a moving conductor (earth-induction apparatus) 

 of known dimensions is made use of; and the strength of the 

 current is found from the deflections of a short magnetic needle 

 surrounded by a multiplier-coil, also of known dimensions. We 

 only require to know in addition the time of vibration of the 



* Abhandl. d. K. Sachsischen Gesellsch. d. Wissenschaften, 1846, vol. i. 

 p. 219. 



t Tom. cit. p. 226. 



