324 



Professor Ewins on Joints 



slightly convex. The deviations from straightness are in 

 neither case more than may well be set down to uncer- 

 tainty in the experimental data ; and taking the two examples 

 together it seems very probable that the joint is equivalent 

 to an air-gap of sensibly constant magnetic resistance. In 











Fig 



2. 







14000 















12000 



go 















_, 10000 



a 



o 



=8 

















» 8000 



el 

 i— i 

















Magneti 



=> o c 









































I 6 8 10 12 14 16 



Magnetizing Force due to Solenoid. 



18 20 



the first example the width of the equivalent gap was about 

 0*0033 cm.: in this case its mean value is about 0*0036 cm. 



Small as these gaps are, it is difficult to believe that the 

 metallic surfaces were actually separated so far, and it seems 

 more probable that the magnetic resistance of a joint is due 

 in part to a reduction of permeability in the metal itself at 

 and close to each surface, as a result of the influence of 

 surface conditions in affecting the grouping of the molecular 

 magnets. 



Since the joint is equivalent to an air-gap of sensibly con- 

 stant width it is easy to apply a graphic construction, like 

 that used by Lord Rayleigh in the case of ellipsoids*, to de- 

 termine the influence which the joint must exert in any cyclic 

 or other magnetizing process ; or, conversely, to determine 

 from experiments made on a circuit in which there is a joint 

 the form which the magnetization curve would assume if the 

 joint were not there. The effect of a gap or of a joint is to 

 make the true magnetic force less than the apparent mag- 

 netic force (namely, the force due to the magnetizing sole- 

 noid) by an amount which is proportional to the magnetiza- 



tion. We have «£>=.£)'— -j-. Thus a curve giving the 



relation of 33 to the magnetizing force in a ring without a 



* Phil. Mag. August 1886, p. 180. 



