264 Prof. G. Wiedemann on the Methods 



exeri upon the needle when the coil is put into rotation with 

 circuit open. In the first experiments of the Committee of 

 the British Association, in which attention had not yet been 

 paid to the separate conditions of the experiment in the way 

 which will be necessary for a final determination of the ohm, 

 these last-mentioned sources of error made themselves in a 

 high degree perceptible. 



Thus F. Kohlrausch, in a discussion of these experiments, 

 has justly pointed out that, in order to avoid inductive action 

 on the coil of wire, the magnet (a magnetized steel ball of 

 8 millim. diameter), in spite of its great mass, had only a 

 moment equal to that possessed by an extremely fine magnetic 

 needle weighing 0*025 gramme. The magnet was attached 

 by means of a wire - 25 metre long to a mirror of 30 millim. 

 diameter suspended by a simple silk fibre of 2 metres length. 

 The currents of air produced by the rotating coil only 0*31 

 metre distant acting upon the relatively large surface of 

 the mirror and magnet, as well as the variable torsion of the 

 thread, become much too considerable in comparison with the 

 directive force of the magnet. 



Further, the vibration of the apparatus produced by the 

 rotation may propagate itself to the case surrounding the 

 mirror, and so put the air, and with it the mirror, into rota- 

 tion. It might thus happen that the deflections obtained by 

 rotating the coil in one or the other direction might vary by 

 as much as 8"5 per cent. If the mean results obtained in dif- 

 ferent series of observations should differ amongst each other 

 by only 2*3 per cent., yet even this is not a guarantee of 

 greater accuracy, but can only be regarded as a proof that 

 the apparatus always acts in nearly the same way. 



Moreover the supplementary elimination of sources of error, 

 e. g. by more exactly calculating the effect of self-induction, 

 as attempted in the memoir of Lord Rayleigh and Dr. Schuster, 

 can in no way free the results from the influence of sources of 

 error shown to exist by the deviations cited above. Above all 

 things, observations of this kind ought never to be arbitrarily 

 corrected on the ground of probability only without having 

 perfectly definite numerical data ; or else all secure experi- 

 mental ground is lost. 



Hence we may consider it shown that the results of these 

 experiments are not to be themselves taken as a final determi- 

 nation of the ohm, but rather as extremely valuable prelimi- 

 nary experiments by which we have become acquainted with 

 the precautions t£> be observed. 



By means of new experiments made by Lord Rayleigh and 

 Dr. Schuster with the apparatus of the British Association, 



