employed for determining the Ohm. 265 



altered in some points, the difficulties mentioned above under 

 (1) remained unaltered ; the magnet, however, was replaced by 

 four magnetized needles, each 0*5 centim. long, fastened on the 

 parallel horizontal edges of a cube of cork. The mirror was 

 attached directly to the cork. But here also the directive 

 force of the magnet is small in comparison with the somewhat 

 powerful influence of currents of air acting on the large sur- 

 face and small moment of inertia. 



Further, the case protecting the suspended portions of the 

 apparatus is attached to the glass tube which surrounds the 

 suspension-thread. Perfect stability could hardly be obtained 

 with such an arrangement ; since with rapid rotation vibration 

 of the mirror would certainly be experienced. These experi- 

 ments also are to be regarded more as preparatory for later and 

 final measurements, and as such have been excellently carried 

 out. On this account no doubt the more exact data for ver- 

 tical adjustment of the axis &c. have not yet been given 

 throughout. Lord Bayleigh, as already mentioned, lays 

 emphasis on the necessity for new coils for the final experi- 

 ments, and raises the question whether these should not be 

 arranged as in the Helmholtz-Graugain galvanometer, so that 

 the directive action of the current on the needle is independent 

 of any small excentricity or deviation of the needle. 



In the final experiments, whether they are made according 

 to this method or according to one of the other methods, 

 it is in any case necessary that as complete a statement should 

 be given of each separate proceeding in arranging and using 

 the apparatus. Also the experiments should not be made 

 with an apparatus set up and adjusted once for all, since then 

 the errors of the arrangement will repeat themselves with each 

 new determination undertaken with the apparatus. On the 

 contrary, the apparatus itself must be frequently altered in 

 various ways. Only so can we obtain results independent of 

 each other, which can be used for mutual control. 



The sources of error which are so difficult to avoid in using 

 method IV., more particularly in consequence of the extra 

 currents, have induced W. Weber* himself, in conjunction 

 with the late F. Zollner, to again take up the first method 

 (which had been employed by the first-mentioned so long ago as 

 1846 in preliminary experiments) with the most perfect experi- 

 mental means. Of two equal coils, weighing some 207 kilog., 

 most carefully and very regularly wound upon mahogany 

 rings, as shown by measurement of different diameters, the 



* W. Weber and F. Zollner, loc. cit. 



