530 MEASUREMENT OF RESISTANCES IN ABSOLUTE VALUE. 



second term is negligable ; it is proportional to the error of ad- 

 justment e, and to the tangent of the inclination. 



The discharge m is determined by a ballistic galvanometer ; if 

 g is the constant of the galvanometer, h the component of the 

 terrestrial field at the point at which it is placed, and a the angle 

 of throw, all corrections made we have 



/&T , H TT 



w =-_ a , or R = 2S^ . 

 g TT h la 



In his first experiments,* as well as in those which he made 

 twice with Z611ner,f and which were completed by Wiedemann,| 

 Weber used two coils of large dimensions, almost identical, one of 

 which served as inductor, and the other as ballistic galvanometer. 



The values of S and of g are determined directly from the 

 dimensions of the coils. If A is the mean radius of the former, 

 a that of the second, N and n the, number of turns, A l and a 2 the 

 values given by the formulas (9) of 727 and (14) of 729, we have 



A* 



We must still introduce in the factor g a correction for the 

 length of the needle (746). 



Weber and Zollner used two bars, the one of 10 cm. and the 

 other of 20 cm. The results obtained with the first exceeded by 

 0-02 those which correspond to the second; they attribute this dif- 

 ference to the difference in length of the needles, and, regarding 

 this error as proportional to the square of the length, they take 

 one-third of this difference for the value of the correction which 

 must be introduced into the results obtained with the bar of 10 cm. 

 This mode of correction seems arbitrary, and but little in agreement 

 with the degree of accuracy of the other measurements. 



TT 



The ratio is determined by making the same needle oscillate 

 in the centre of the two instruments. 



* W. WEBER. Elektrodyn. Maasbestim.Abhandl. der Konigl. Sachs. Ges., 

 Vol. i., 'p. 219. 1846. 



t W. WEBER and ZOLLNER. Berichte der Kon. Sachs. Ges., Vol. n., p. 77. 

 1880. 



% G. WIEDEMANN. Versuche zur Bestimmung des Ohms. 1884. 



