544 A NEW CURRENT WEIGHER, ETC. 



noteworthy that, to this approximation, no correction is to be applied to the axial 

 length of the coil. The argument applies also to the fixed coils of the balance. 



For " natural " distribution of current in the coils of the ampere balance, the 

 force F calculated on p. 509 is too great. The corrected value of the force is 



8ada 





where and A are the mean radii of the suspended and fixed coils. We may 

 write (11) in the form 



a L <M_a.dF , A<M A 



" M da '~~~ F ' da q '''' M dA. '' '' F ' 



where 



Inserting the values of R, q, r, A, and a, tabulated on pp. 488, 512, 513, we obtain 

 a correction to the force of 17 parts in 10,000,000 for the complete system of coils. 

 The sign of the correction is negative. 



If we assume the current density to be uniform, the force in this case is found by 

 making u = i/TrR 3 in (9), and the corrected value of the force is 



F 



Badet 8A c/A ' 



The correction is of the same value as before, but of opposite sign. As the distri- 

 bution of current is uncertain, the value of the force stated on p. 509 lias been used 

 throughout our work. 



