A NEW CURRENT WEIGHER, ETC. 



503 



force is equal to 0*14 dyne when the current is 1 ampere and circulates in the same 

 direction through all the coils of the system. The corresponding force for the other 

 system is 0'34 dyne. The value of the force for a displacement d mils from the correct 

 axial position when 1*02 amperes is passing is given by the expression 4'8 x lQ~*dg dynes, 



O lO'O 2dO 3OO 4OO 



CHANGC OF FORCE IN OVKJCS (CUR . I'OS AMP.j 



Fig. 18. Change of force due to axial displacement of coils. 



where g = 981 centiins./sec*. The force is also given by O'ldxg, where a; is in centi- 

 metres. Thus, for an axial displacement of 1 mil of the fixed or suspended coils, a 

 change in balancing mass of 0*48 milligramme results ; the axial position may there- 

 fore be fixed to less than 1 mil. 



(2) Setting of the Fixed and Suspended Coils to be Concentric. When the coils 

 are coaxial the mutual induction is a minimum with respect to radial position, and 

 when the current flows in opposite directions in the upper and lower fixed coils the 

 force changes with displacement from the coaxial position. The change in force with 

 radial displacement was measured for both fixed cylinders, the displacements being 

 made in two directions at right angles ; the results were plotted in four curves, of 

 which two (those for the- left fixed cylinder) are given in fig. 19. Inspection shows 

 that the rate of variation of mutual induction with radial displacement increases with 



