A NEW CURRENT WEIGHER, ETC. 517 



., 



Cylinders. When the currents in the a and 6 wires on the left suspended cylinder 

 are in opposition, the maximum force due to the current in the left fixed helices is 



y'y k {(M ula + M c * + M LIa + MU.) - (M Dlt + M t ,+ M LI4 + MU,)}. 



The difference of the mean diameters of the helices on the left suspended cylinder 

 was measured as 3/x and the difference of those on the right as 2/i. For a mean 

 difference in radius of 1'2/t the value of the force for one system is O'OG dyne. If the 

 left and right systems are made to co-operate, the necessary change in mass to 

 maintain equilibrium when a current of 1*02 amperes is passed through all the coils 

 and reversed in the fixed coils should be 0'25 milligramme. The experimental value is 

 0'3 s g l milligramme; corresponding to a difference in the mean diameters of 3'5/x I/A. 



Change of Relative Azimuth of Fixed and Suspended Cylinders. Lord RAYLEIOH 

 has pointed out* that the value of the mutual induction of two coaxial helices is 

 dependent on the relative position of the helices, and that in strictness both helices 

 cannot be replaced by current sheets. The complication thence arising can be 

 eliminated in experimental applications by a relative rotation, since the mean field is 

 strictly symmetrical, and accordingly the mean mutual induction is the same as if 

 both helices were replaced by current sheets. 



The fixed and suspended coils of the ampere balance are normally arranged, so that 

 the diametral plane containing the termini of the fixed coils on one cylinder is 

 practically coincident with that containing the termini of the coaxial suspended 

 coils. The mutual induction must be slightly different when these planes are at 

 right angles, and attempts were made to estimate this difference by experiment. 

 The difference of the forces exerted by the left and right systems was first determined 

 in the manner indicated on p. 515. One set of fixed coils was then turned through 

 90 and the difference again measured ; there was no certain change in the difference, 

 and had the change in mutual induction been as great as 5 in 1,000,000 it must have 

 been detected. The fixed coils of the other system were then turned through 90 

 and the difference in mutual induction of the two systems again determined ; it agreed 

 with the previous results. The angle was altered to 60 and a few more measure- 

 ments made, but no change in the difference was observed. The complete set of 

 observations lead us to conclude that the mutual induction of the helices does not 

 vary with change in the orientation of the coils by more than 1 in 1,000,000. 



SECTION 12. USE OF BALANCE AND DETERMINATION OF E.M.F. OF 



CADMIUM CELL. 



The arrangement of the circuits employed in the determination of current strength 

 and of the E.M.F. of the standard cell is shown diagrammatically in fig. 22, and in 

 further detail in fig. 23. Fig. 24 gives a general view of the apparatus as used. 



* B.A. Report,' 1899, p. 292 (Report of Electrical Standards Committee). 



