468 PKOFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



determination of the electro-chemical equivalent of silver.* In this instrument 

 the coils were formed by winding insulated wire in the grooves of screw threads 

 cut in metal cylinders, but the springiness of the covering prevented very exact 

 measurements of the dimensions being made. To obtain greater precision, it was 

 decided to use, in the proposed balance, bare wire wound on insulating material, 

 as originally employed in the Lorenz apparatus designed by Professor J. V. JOM.S 

 for the McGill University, Montreal, and to avoid the uncertainty as to leakage 

 between adjacent turns of such a spiral t the arrangement devised by one of the 

 authors (W. E. A.) of having double-threaded screw grooves wound with separate 

 bare wires, subsequently connected in series after the insulation resistance between 

 them had been made satisfactory, was adopted. 



Experience with the preliminary apparatus showed that air convection currents 

 should be minimised, and that easy access to, and independent adjustments of, both 

 fixed and suspended coils were very desirable. In designing the new current weigher, 

 in collaboration with the late Professor JONES, these points were kept in view, and 

 the arrangements chosen were such as would take full advantage of the mechanical 

 precision attainable with modern machine tools, a subject which Professor JONES had 

 very much at heart. In fact, he had long advocated that the instruments employed 

 in realising the concrete values of the electrical units from their absolute definitions 

 should be engineering tools rather than ordinary physical laboratory apparatus. 



Complete working drawings and specifications of the proposed instrument, and its 

 adjustable support, were prepared at the Central Technical College during the Session 

 1898-99, the drawings being made by Mr. J. P. GREGOEY, then a student of the 

 College, and now of the British Thomson Houston Co., Rugby. Tenders were 

 obtained for the construction of the instrument, to defray the cost of which the 

 British Association for the Advancement of Science made a grant of 3004 



As the amounts of the tenders for the balance, and the adjustable phosphor-bronze 

 stand for supporting it, much exceeded the above-named sum, Sir ANDREW NOBLE, 

 F.R.S., was approached, and took so much interest in the apparatus and the important 

 work that was to be carried out with it, that he generously presented the carefully 

 made adjustable support, constructed by Messrs. SIR W. ARMSTRONG, WHITWORTH 

 and Co., Limited, free of cost. 



The physical balance was built by Mr. L. OERTLING, of London, and the electrical 

 portions were made at the National Physical Laboratory, under the supervision of the 

 Director, Dr. R. T. GLAZEBROOK, F.R.S. 



We may here remark that the current weigher has proved to be the most perfect 

 absolute electrical instrument hitherto constructed, and has enabled us to determine 



* 'B.A. Report,' Bristol, 1898, p. 157; also 'Jour. Inst. Elec. Engrs.,' vol. 35, p. 12, 1905. 

 t This uncertainty necessitated the removal of the original winding of the Lorenz apparatus, and 

 rewinding with silk-covered wire. See ' Jour. Inst. Elec. Engrs.,' vol. 35, p. 1 3. 

 J 'B.A. Report,' 1898, p. 147. 



