A NEW CURRENT WEIGHER, ETC. 467 



in 1904, a now torsion electrodynamometer of the GRAY* pattern, having single-layer coils on cylinders 

 i if Paris plaster. Experiments on CLAKK and cadmium cells were then in progress. 



Last year (1906) GuTHEf published the results of a lengthy research on CI.AKK and cadmium cells in 

 which another GRAY electrodynamometer was employed. He arrived at the values 1*43296 at 15" C. and 

 1 -01853 at 20 C. for the respective cells, and deduced from this and previous work 1 '11773 milligrammes 

 per coulomb as the electrochemical equivalent of silver. The instrument employed by GITHE suffers from 

 non-uniformity of winding, but this was allowed for approximately. Its influence on the accuracy of the 

 electrodynamometer is discussed by ROSA in the same number of the ' Bulletin ' (p. 71). 



SECTION 1. INTRODUCTORY. 



The instrument herein described is the outcome of conversations between the late 



I'lnt'rssnr -I. YllMAMU .li'M-ls ami MM.- of (lie :tlltliol> I W. K. A.) '!! tllril' ivtlini tVoln 



the British Association meeting, held in Toronto in 1897. 



Absolute determinations of resistance had been made on many occasions, and with 

 considerable precision, whilst those of current were comparatively few ; the want of 

 agreement between the results obtained by different observers was by no means 

 satisfactory. It was therefore decided to make a new determination of the ampere by 

 means of a current weigher formed of coils with single layers of wire, such as had been 

 so successfully employed by Professor JONES in his determination of the " Specific 

 Resistance of Mercury in Absolute Measure" ('Phil. Trans.,' A, 1891), and by 

 Professors AYRTON and JONES in their determination of the ohm at the Central 

 Technical College, London, in 1897.J 



By using coaxial coils, with single layers of wire wound in screw-thread grooves, 

 advantage could be taken of the convenient formula developed by Professor JONES for 

 calculating the electro-magnetic force between a helix and a circular current sheet, 

 viz., 



where y k is the current in the helix, y the current per unit length of the current 

 sheet, and M,, M a the coefficients of mutual induction of the helix and the two circular 

 ends of the current sheet. 



To test the stability of the proposed current weigher, or " ampere balance " 

 as it is frequently called, as well as to get experience regarding the conditions 

 necessary for successful operation, a preliminary apparatus was constructed at 

 the Central Technical College in 1898, and there used to make an approximate 



* ' GRAY'S Absolute Measurements, &c.,' vol. 2, part 1, p. 274. 



t ' United States Bureau of Standards Bulletin,' vol. 2, No. 1, p. 33, 1906. 



t 'B.A. Report,' Toronto, p. 212, 1897. 



" On the Calculation of the Coefficient of Mutual Induction of a Circle and a Co-axial Helix, and of 

 the Electromagnetic Force between a Helical Current and a Uniform Co-axial Circular Cylindrical Current 

 Sheet," 'Roy. Soc. Proc.,' vol. 63, p. 204, 1898. 



3 o 2 



