employed for determining the Ohm. 267 



It is a point of very great importance in the employment of 

 this method that self-induction in the inductor is without in- 

 fluence, and that, further, the intensity of the earth's mag- 

 netism does not enter into the calculation, provided that it is 

 of equal intensity in the positions occupied by inductor and 

 multiplier. It may not be allowable to assume that this is 

 the case in ordinary rooms, because of iron hooks let into the 

 walls ; hut in a building constructed for the purpose this con- 

 dition might easily be fulfilled. This point may also easily be 

 controlled by oscillation-experiments ; the ratio of the directive 

 force of the earth may be determined at the two places. 



It is true that we have to set against this advantage that we 

 have two coils to measure, and are so liable to a double error. 

 (This, however, is also the case in the experiments of the Bri- 

 tish Association described above, where the revolving coil con- 

 sists of two separated halves.) This disadvantage may be 

 considered small in comparison with the error resulting in 

 the fourth method from the measurement of the self-induction 

 of the coil. It is further reduced by the large dimensions of 

 the coils. At the same time stability of the apparatus, and 

 consequent freedom from disturbances resulting from vibration 

 and from currents of air, is much more easily obtained than 

 with the rapidly -rotating coil employed in the former method. 



Previous communications on experiments with this appa- 

 ratus are only preliminary, on which account we are not yet 

 in possession of details of adjustment ; for which, however, 

 not only the name of W. Weber, but also that of Eepsold, the 

 instrument- maker intrusted with the work, give us full gua- 

 rantee that such points as the vertically of the axis of rotation, 

 the adjustment of the coil magnetic east and west, the arrange- 

 ment for displacing the coil through exactly 180°, and so on, 

 will be attended to. The preliminary method of measuring 

 the circumference of the mahogany rings on which the coils 

 are wound and of their exterior circumference by means of 

 strips of paper, which were then compared with a divided 

 wooden rule, will no doubt give place to more exact methods 

 in the actual experiments. 



A special advantage of this method is that the wires may 

 be unwound from the rings and wound again without any 

 great difficulty; and so repeated observations independent of 

 each other can be obtained. 



This method is therefore to be recommended for the final 

 determinations. 



If we employ Weber's second method, we have no need to 

 determine the dimensions of the multiplier ; but we determine 



