Magnetic Permeability of an Iron Bar. 281 



almost as a solid bar. Facing the surfaces truly does not of 

 itself do away with the apparent air-space : so long as no load 

 is applied the apparent space remains. But when the surfaces 

 are faced true and the parts are strongly pressed together, the 

 " magnetic resistance " of the joint practically vanishes. 



The following table shows the induction and the thickness 

 of the apparent air-space when the surfaces were faced to true 

 planes, and the bar was not loaded. 



Table III. — Cut. Bar with True-plane Surfaces. 



Magnetizing 

 force, §. 



Induction, 35. 



Thickness of 

 equivalent 

 air-space, in 

 millimetres. 



Solid bar. 



Bar cut and 

 faced. 



5 

 10 

 15 

 20 



G300 

 11000 

 12900 

 13900 



4300 



9000 



12000 



13450 



0058 

 0026 

 0012 



0-006 



In this example the apparent air-space between the faced 

 surfaces at the cut vanished completely under the applica- 

 tion of load, as is shown by the way in which the points 

 marked thus x (which refer to operation No. 6) fall very 

 exactly on curve 2. The cut and faced bar, loaded with 226 

 kilogs. per square centim., could not be distinguished as to 

 permeability from the same bar in its uncut state when thai* 

 was tested under the same load. 



The above result was of so much interest, that the experi- 

 ment of magnetizing a bar under load after the bar had been 

 cut and the surfaces had been faced true was repeated on two 

 other bars. In neither case was the apparent air-space so 

 completely annihilated as it had been in the first instance, by 

 the combined effect of load and true facing ; but the general 

 result was confirmed, that a bar prepared in this way and 

 sufficiently loaded was not materially different in its magnetic 

 properties from an uncut bar. Fig. 4 shows the results of 

 another group of tests similar to those of fig. 3, but made 

 with a new bar. Curve aaa \s for the solid bar without load; 

 bbb is for the same bar cut and faced, but not loaded ; ccc is 

 for the solid bar, and d<l <1 for the cut bar, loaded in each case 



