employed for determining the Ohm. 269 



employing a magnet of 80 millim. length, 20*1 millim. breadth, 

 and 21*1 millim. depth, suspended by a silk thread between 

 two conical coils very regularly wound, of 144-43 millim. in- 

 ternal, 184-46 millim. external radius, and 51*64 millim. 

 breadth. 



The mean distance of the spirals from each other was either 

 very small, or the distance of their median planes was made 

 164-4 millim., or very nearly equal to the mean radius of the 

 coils. In this last case the distance of the poles from each other 

 has no important influence, although it has in the first, where 

 also the distribution of moments in the magnet must not be 

 neglected. We must, however, always take into account 

 inevitable differences in measurement in consequence of the 

 mutual pressure of the coils and certain unavoidable irregu- 

 larity in the winding, which are more important in consequence 

 of the small dimensions of the coils, as in the fourth method 

 which we have already considered. Nevertheless it was shown, 

 by comparison with a standard resistance of Siemens^, with 

 which the resistance of the coils was compared by means of a 

 "VTheatstone's bridge, in both the cases mentioned, and after 

 rewinding of the coils, that the standard resistance expressed 

 in absolute measure varied in three series of experiments only 

 from 0-9532 to 0-9570, from 0-9528 to 0-9555, and from 0-9527 

 to 0*9551 — that is, not more than 0*5 per cent. The mean is 

 0-95451. 



In further experiments according to another method, Fr. 

 "Weber placed the coils mentioned above at a definite distance 

 from each other, connected the one, " the inducing coil," with 

 a very constant DanielPs cell and a simple ring of 168*7 

 millim. radius; the other, " the induced coil," with a multi- 

 plier, which was composed of a coil consisting of two equal 

 conical halves placed close together, of 154*2 millim. inner 

 radius and 172 - 2 external radius, between which was suspended 

 a magnet 40 millim. long provided with a mirror. The simple 

 ring of the inducing circuit lay between these coils. The 

 deviation of the needle whilst a constant current passed through 

 the inducing circle measured its intensity J, w T hich was to be 

 measured in absolute measure from the known dimensions of 

 the ring ; the deviation on breaking the inducing circuit after 

 disconnecting the ring gave the intensity i of the induced cur- 

 rent. Since the induced electromotive force e= JP (where P 

 is the mutual potential of the coils) may be calculated in abso- 

 lute measure from the dimensions of the coil by putting the 

 constant of induction equal to unity, we obtain the resistance of 



the induced circuit r from the formula r = -. 



