320 Professor Ewing on Joints 



Accordingly 



1 4 



p ~ -274 x e™' 



This may be multiplied by 4 to represent the increase of 

 effect in the actual circumstances as compared with what is 

 supposed in the two-dimensional problem ; but it remains 

 absolutely insensible in comparison with the other curvatures 

 involved. 



XXXVI. On Joints in Magnetic Circuits. 

 By Professor Ewing, M.A., F.R.S.* 



IN the ' Philosophical Magazine ' for September 1888 an 

 account was given by Mr. William Low and myself of 

 experiments we had made to examine the influence of a plane 

 of transverse section in an iron bar which formed part of a 

 magnetic circuit. It was found that a transverse cut intro- 

 duced what is now frequently spoken of as magnetic resist- 

 ance, even when the faces at the cut were scraped up to be 

 as nearly as possible true planes, and were brought into ex- 

 cellent mechanical contact. Only when a very considerable 

 force was applied to press them together did the resistance 

 of the joint appear to vanish ; except in this case it required 

 a stronger magnetomotive force — a greater number of ampere- 

 turns in the magnetizing coil — to bring the magnetic induc- 

 tion up to any assigned value when the bar was cut than it 

 had required when uncut. In this respect the joint is equi- 

 valent to a narrow crevasse of air or other non-magnetic 

 material. We proceeded to calculate the width of the air- 

 gap which should be equivalent, in magnetic resistance, to 

 the joint, and obtained values which made it appear that the 

 width of this equivalent air-gap became reduced when the 

 magnetization was forced up to high values. 



Having occasion recently to revise these results I found 

 that an incorrect procedure had been followed in calculating, 

 from the experimental data, the width of the equivalent air- 

 gap. When the necessary correction is made it appears that 

 this width remains constant, or nearly constant, whether the 

 magnetization be weak or strong. The object of this note is 

 to point out the error in the former calculation, and to state 

 the corrected values. 



Suppose we have to deal with the magnetization of a com- 

 plete iron circuit, of uniform cross section, and of length I. 

 Let the permeability of the iron be //<, for any induction 23. The 



* Communicated by the Author. 



