Soft Iron, especially with weaker decomposing -powers. 53 



easily be seen, in opposition to the primary current ; while the 

 action of the extra currents induced in P changes its direction both 

 with the initial placing of C, and with the placing of C 2 . Hence 

 arise small movements of the magnet, the direction of which 

 changes with the placing of C 2 , but their magnitude with the 

 nature of the change of position of Cj. This was actually ob- 

 served. It is not difficult to estimate the influence of this source 

 of error both upon the calculation of A and on the values of a 

 (when these are determined from multiplication-readings), and 

 to apply the correction. Moreover the correction of A can be 

 dispensed with, if the secondary wire be inserted in the multi- 

 plier now in one direction and then in the opposite. 



As an example of the method here discussed, I cite, abridged, 

 the record of one measurement : — 



\9th October 1871 ; Series, 12 Daniell's elements weakened 

 by 10 Siemens' s resistance. Roll at 1000 millims. 



Number of primary turns »' = 250 -200 + 150 -100 =100. 



Temperature at the ring, / r = 16 0, 7 0. ; temperature at the 



multiplier, t m = 9°'7. Hence W = 11-874 x 10 10 ^^. 



Distance of the scale from the mirror of the magnet =2125*3 

 scale-divisions. 



Logarithmic decrement X = 0*1410 ; consequently 



log^ arctau X = 0-06588. 



Duration of an oscillation of the magnetometer undamped, 

 T = 20-419 seconds. 



Direction-force of the same, H = 1*9820 milligr.^ rnillim. - * 

 sec.— 1 . 



The eight positions of equilibrium of the magnet (with three 

 combinations of C x and C 2 and both ways of connexion of the 

 secondary wire with the multiplier) were, corrected for the final 

 distance of the scale from the mirror : — ■ 



621-5 404-8 



622-2 406-2 



623*3 405-9 



621-7 405-4 



Means . . 622*17 405-57 



The corresponding movements of the magnet through induc- 

 tion-shocks were ; — 



243-4 244-4 



241-7 244-5 



241-9 214-4 



243-7 244-4 



