268 Prof. G. Wiedemann on the Methods 



the damping of the oscillations of the needle, once with open 

 circuit and again with closed circuit. Hence, by determining 

 the deviation of the needle when the inductor is displaced, 

 the resistance of the circuit in electromagnetic measure may- 

 be calculated. For this purpose the multiplier must enclose 

 the needle more closely, so that the damping may be sufficient. 

 We have to determine, besides the dimensions of the inductor, 

 the period of oscillation and moment of inertia of the needle 

 and the intensity of the horizontal component of the earth's 

 magnetism. The sources of error of this method have been 

 carefully examined by F. Kohlrausch*; the error in measure- 

 ment of the inductor may, however, have been scarcely esti- 

 mated high enough. (Compare also the following method.) 



If proper experimental means and a suitable observatory 

 are availablef for accurate determination of the earth's mag- 

 netism, this method may very well be employed together with 

 Weber's first method. 



In Weber's third method, which is apparently so simple, we 

 have, in order to determine the absolute resistance of a multi- 

 plier, to observe only the damping of the oscillations of a 

 needle suspended in the coil when the ends of the coil are 

 united, and again when the circuit is open. In order to be 

 able to calculate the electromotive force due to the oscillations 

 of the needle and its action upon the needle, which is directly 

 proportional to that electromotive force, and inversely propor- 

 tional to the resistance to be measured, we must know in 

 general the dimensions of the multiplier and the position of 

 the needle relative to it, as well as the distribution of the 

 magnetism in the needle. 



The former determinations are difficult to carry out exactly, 

 since, in order that there may be sufficient damping, no very 

 large dimensions can be given to the coil ; the latter determi- 

 nations can only be made inexactly by observation of the 

 currents induced in a short coil at different points of the 

 magnet, or by numerous oscillation- and deflection-experiments 

 with a magnetic needle swinging at different distances from 

 various points of the magnet. We have also to satisfy our- 

 selves that the damping is not dependent upon the angle of 

 deflection of the needle. 



This method has been employed by Fr. Weber J, in Zurich, 



* Loc. cit. 



t Compare also the new methods by F. Kohlrausch, Gottinger Nach- 

 richten, March 4, 1882. 



\ F. Weber, Elektromagnetische unci halorimetrische Mesmngen (Ziir- 

 cher und Furrer, Zurich, 1878) ; Beiblatter, vol. ii. p. 499. 



