XII. A New Ctirri'iit H'eif/ht-r and a Determination of the Electromotive Force 



of the Normal Wexton Cadmium 



Hi/ /',<,/, .,-.>/ W. E. AYRTON, F.K.S., and'L MATHER, F.R.S., Central Technical 



Coller/<\ famitnn; and F. E. SMITH, A.R.C.S., National Physical 



Laboratory, Tcddington. 



Received June 5, Read June 27, IU07. 







[PLATES 7-8.] 



PRINCIPAL CONTENTS. 



1'age 



lintorical notes on the absolute measurement of current 464 



Introductory 467 



(General description of current weigher 469 



Adjustable support for balance 471 



The physical balance 472 



Magnetic tests 475 



Construction, measurement and insulation of coils 478 



Axial lengths of coils 483 



Diameters of coils 486 



Insulation of coils 496 



Erecting and adjusting the instrument 499 



Advantages of duplicating the coils 505 



Force between helical current and coaxial circular current sheet 507 



Calculation of mutual induction of helix and circular end of coaxial current sheet 510 



Differential t'ffects of the several windings and their relation to the linear dimensions of the 



coils 615 



Use of balance and determination of E.M.F. of cell 517 



Preliminary difficulties 523 



< icneral behaviour of the balance 527 



Tables of results and discussion of same 529 



History of the standard cell employed 535 



Conclusions 538 



Appendix A. Coefficients for calculation of the complete elliptic integrals F and E 540 



Appendix B. On the forces between coils of wire of finite section 54 1 



VOL. CCVII. A 424. 31.1.08 



