Magnetic Permeability of an Iron Bar. 

 Table IV. 



283 



Magnetizing 

 force, Jp. 



Induction, 23. 



Thickness of 

 equivalent 

 air-space, in 

 millimetres. 



Solid bar. 



Bar cut and 



surface faced 



true. 



4 



3950 



3000 



0-042 



6 



0900 



5300 



0034 



8 



9250 



7400 



0-028 



10 



10900 



9150 



0023 



15 



13250 



12000 



0016 



20 



14300 



13500 



0-011 



30 



15200 



14900 



0005 



Tests made with a third bar gave results which were in close 

 agreement with those that have just been described. In this 

 last case comparative measurements were made of the effects 

 of various values of external load in causing the apparent air- 

 space between well faced surfaces to disappear. The bar, first 

 before being cut and again after being cut, and after having 

 its cut surfaces scraped into true planes, was magnetized under 

 first one and then another of a series of compressing loads ; 

 the highest being of the same value as in former cases (226 

 kilogs. per square centim.), and a curve of 53 and § was found 

 for each. As in the second example described above, this 

 highest load did not succeed in altogether obliterating the 

 apparent air-space except for magnetizing forces ranging up 

 to about 5 C.G-.S. units, and again for high forces. And loads 

 less than the highest failed to make the air-space vanish, even 

 under small magnetizing forces. To compare the effects of 

 different loads, it will suffice to give the induction before and 

 after cutting which was reached by applying a magnetizing 

 force of 5 U.G.S. units to the bar, when the load had the 

 successive values shown in Table V. 



With this bar a supplementary experiment was made at 

 the suggestion of Sir William Thomson, to examine the effect 

 of introducing a film of gold-leaf between the (true-plane) 

 faces of iron at the cut. It was found that when the pieces 

 were not pressed against each other by an external load, the 

 presence of the gold-leaf made no sensible difference in the 

 magnetic "resistance" of the joint; when the pieces were 

 pressed together with a force equivalent to 220 kilogs. per 

 square centimetre, the presence of the gold-leaf did slightly 



