236 M. W. Weber on the Measurement of Electric 



The mean value of these four series is 79*755 parts of the 

 scale =79*4 rnillims., which must be increased by ^ a millim. if 

 we are to take into account the influence of the fact that the 

 rotation of the ring A cannot be effected in a time so small that 

 it can be neglected in comparison with the time of oscillation of 

 the needle. From this we obtain for a the value 



_ 79-9 

 a ~ 8175' 



inasmuch as double the horizontal distance of the mirror from 

 the scale is exactly 8175 millims. 



The time of oscillation of the needle was found from 300 

 oscillations to be 



* = 10"-2818, 



in which the part of the directive force arising from the elasticity 

 of the thread was the 1770th part of the magnetic directive force, 

 and hence 



1 _ 1770 

 1 + ~1771' 



Finally, on account of the great distance of the two rings in a 

 room not free from iron, the time of oscillation of the same 

 needle was compared for the position of both rings, and their 

 ratio found to be as 2 - 9126 : 2*9095 ; from which it follows that 

 if T' is the terrestrial magnetism for A, T" for B, we have 



T':T" = 470:471. 



These observations are sufficient for determining the resist- 

 ance of the whole closed conductor ; and by accurate calculation 

 we get the value 



w = 2166-10. 



§ 4. Application of the principle of Deadening . 



Instead of using terrestrial magnetism to obtain an electromo- 

 tive force which can be referred to an absolute measure, bar mag- 

 netism may be employed ; in that case it is obvious that the most 

 convenient position for the bar-magnet whose magnetism is to be 

 used, will be in the centre of the ring formed by the closed con- 

 ductor. The magnet may then either be fixed, and the ring 

 moveable about its diameter at right angles to the magnetic axis 

 of the bar; or inversely, the ring may be fixed and the magnet 

 moved backwards and forwards about that diameter. In the 

 latter case a strong oscillating magnetic needle may be used, sus- 

 pended in the centre of the ring. 



