Determination of Resistances in Absolute Measure. 335 



difficulty, i. e. the unsteadiness of the suspended mirror, all 

 that is then necessaiy to know with accuracy being the dis- 

 tance between the two scales. 



In using this and the three following methods great pains 

 must be taken with the levelling of the earth inductor, since 

 the deviation of the axis of rotation from the vertical (at least 

 in the plane of the meridian) gives rise to an error of the first 

 order with (in these latitudes) a high coefficient. In this 

 respect it would be a decided advantage to carry out the 

 experiments in a locality nearer to the magnetic equator (see 

 "Account of Experiments to determine the value of the B.A. 

 Unit in Absolute Measure," Phil. Trans, for 1882, p. 684). It 

 is to be hoped that the measurements commenced by Weber 

 and Zollner will be carried to a successful issue, as it is only 

 by the coincidence of results obtained by various methods that 

 the question can be satisfactorily settled. At present no value 

 in absolute measure of the B.A. unit or of the Siemens unit 

 has been published as the result of their work. 



III. Method of Revolving Coil. 



This method, first, it would appear, suggested by Weber, 

 was carried into execution by the celebrated Electrical Com- 

 mittee of the British Association*, and more recently by 

 myself with the assistance of Dr. Schuster and others f. The 

 greater part of what I have to say upon this subject has been 

 put forward already in the papers referred to, from which 

 alone the reader can form a complete opinion on the merits 

 or demerits of the method as hitherto practised. On the pre- 

 sent occasion I must take many of the conclusions there arrived 

 at for granted, or at most give a mere indication of the nature 

 of the arguments by which they may be supported. 



Method III. differs from II. mainly in the fact that in III. 

 the earth-inductor is, so to speak, its own galvanometer, the 

 needle whose deflections measure the currents being suspended 

 at the centre of the revolving coil itself instead of at the centre 

 of another galvanometer-coil forming part of the same circuit. 

 If, as in II., the inductor-coil were simply twisted through 

 180° when the needle passes its position of equilibrium, the 

 disadvantages of the simplification would probably prepon- 

 derate over the advantages. The diminution of effect due to 

 the oblique position of the coil relatively to the needle (except 

 at the moment of passing the magnetic meridian) would indeed 

 be compensated by the diminished resistance of the complete 

 circuit, and, as will presently appear, considerable advantage 



* Brit. Assoc. Eeports, 1862-1867. Keprint, Spon, 1873. 

 t Proc. Eoy. Soc. May 1881, Feb. 1882 ; Phil. Trans. 1882. 



