TRANSACTIONS OF SECTION A. 609 



4. Final Value of the B.A. Unit of Electrical Resistance as determined by 

 the American Committee. By Professor H. A. Rowland. 



5. On the Specific Resistance of Commercial Iron. 

 By W. H. Preece, F.B.S. 



The Swedisli iron now used for telegraph wire has a specific resistance of 6-034 

 instead of 6'558 as given in text -books. The specific resistance at 60° F. is 



Silver . . . 1-609 I Copper . . . 1-642 

 Pure Iron . . 0-753 | Commercial Iron . 9-886 



The wire now supplied has a conductivity of 98-44 per cent, of pure iron. 

 The temperature coefficient is given in the formula 



R„ = R, (1-0048)'-'. 



6. On the Influence of a Plane of Transverse Section on the Magnetic 

 Permeability of an Iron Bar. By Professor J. A. EwiNG, B.Sc, F.B.S., 

 and William Low. 



It has been remarked by Professor J. J. Thomson and Mr. H. F. NewaU that 

 when an iron bar is cut across, and the cut ends are brought into contact, the 

 magnetic permeability is notably reduced.^ The attention of the authors was 

 directed to the matter by finding the same phenomenon present itself in experiments 

 on the magnetisation of iron by the ' isthmus ' method, and they proceeded to 

 examine the eflect by an application of the method Hopkinson has used to measure 

 magnetic permeability.'^ A round bar, nearly half a square centimeter in section, 

 and 13 cms. long, had its ends united by a massive wrought-iron yoke to reduce it 

 to a condition approximating to endlessness; and its magnetisation by various 

 magnetic forces was examined, both when free from stress and when compressed by 

 a load of 226 kilos per sq. cm. It was then cut in the lathe, the halves placed in 

 ■contact, and the magnetism again examined with and without load. It was next 

 cut into four parts, and finally into eight parts, and magnetised in each case. 



Every new plane of section caused a notable loss of permeability. The following 

 are the maximum values of the permeability in each case : — 



Solid bar . . 1220 1 Bar cut in two . . 980 



Bar cut in four . 640 | Bar cut in eight . . 400 



Next another bar was tested, first, when solid ; next with one cut finished in the 

 lathe; and finally with the cut surfaces faced true by scraping and comparing them 

 with a "Whitworth plane. So long as the bar was nut compressed, its magnetic 

 permeability was nearl}' the same, whether the ends were left roughly finished or 

 "were faced true. But when load was applied the effect of facing the ends was 

 remarkable : the faced bar then behaved as a solid bar would, while the bar with 

 rough-cut ends still showed a decided defect of permeability as compared with the 

 solid bar. 



This made it seem highly probable that the whole effect was due to a film of 

 air between the cut faces. Applying Hopkinson's method to calculate the thickness 

 this film would need to have, in order to account for the observed increase of 

 magnetic resistance, the authors find its thickness is only about /j of a millimeter 

 when the magnetic force is 10 c.g.s. units, and diminishes to about ^tj of a 

 millimeter when the force is 50 c.g.s. units. In the case of the bar cut into four 

 and eight parts, each cut has an eff'ect equivalent to the introduction of a film of 

 this thickness. The authors conclude that in all probability the whole phenomenon 

 is due to the surfaces being separated by these short distances. 



' Cambrith/c Phil. Soc. Proc, Feb. 1887. 



^ ' Magnetisation of Iron,' PJiil. Trans, part ii. 1885. 



1887. R a 



