A NEW CURRENT WEIGHER, ETC. 507 



From the curves it will I* seen that with no current through the coils, or with 

 normal current through both sets, the drift was comparatively small, Amounting in 

 the worst case to only 0'15 division (0'014 millim.) in five complete periods. With 

 current through one set only, however, the drift was much greater, amounting to 

 076 division in five periods in the lowest curve, the direction being such as to 

 indicate increase in weight of the suspended coil through which the current was 

 flowing. As the sensitiveness of the balance during the above tests was 0'82 division 

 for the reversal of 1 milligramme, the apparent rate of change of mass amounted to 

 0'38 milligramme per period (or 0'65 milligramme per minute), when current passed 

 through one set of coils only, whilst with current through both sets the greatest 

 change was about a fifth as great. There is, therefore, a considerable increase in 

 steadiness of the rest-point when both sets of coils are used. 



Other advantages of two sets of coils are (a) that two independent determinations 

 of the ampere can be made by using the sets separately ; (b) the two sets being very 

 nearly alike, one serves as a check on the constancy of the other set by arranging 

 them in opposition and weighing the difference between their effects, which difference 

 should, of course, be constant for a given current ; (c) the difference in the force, if 

 any, produced by changing the relative positions in azimuth of the fixed and suspended 

 helices as suggested by Lord RAYLEIGH* can be readily found by making the 

 differential test above mentioned with one set of coils in a certain relative position, 

 whilst that of the other set is varied. The result of such a test is given on p. 517, 

 Section 11. 



A lengthy experience with the current weigher proves that the self-checking 

 facilities provided in the instrument are of very great utility and form one of the 

 most valuable features of the balance. 



SECTION 9. FORCE BETWEEN HELICAL CURRENT AND COAXIAL CIRCULAR 



CURRENT SHEET. 



As mentioned in the introductory section (p. 467) of this paper, the formula used 

 for calculating the force between the fixed and suspended coils is due to the late 

 Professor J. V. JONES, viz. : 



M,) .......... (1), 



the meanings of the quantities being as there defined. This formula is rigorously 

 exact for a helix and current sheet, and a very close approximation for two helices 

 of fine pitch. The order of the error is considered in Appendix B, p. 541. 



The arrangement of the coils in the actual instrument may be represented 

 diagrammatically in section by fig. 21, which is meant to indicate a vertical section 

 through the vertical axes of the windings, the vertical dotted lines being the axes of 



the coils. 



* 'B.A. Report,' Dover, 1899, p. 292. 



3 T 2 



