in Magnetic Circuits, 321 



line-integral of magnetic force producing this induction is 



— , when there is no joint. 



Next suppose a joint to be introduced and to be equivalent 

 in magnetic resistance to an air-gap of width x. The line- 

 integral of magnetic force is then 



Distinguish by dashes the values of 33 and p in this case 

 from their values when there was no joint. Then, taking the 

 same number of ampere-turns in the magnetizing coil in 

 both cases, we have 



from which 





*\ 



In the paper referred to we omitted to take account of the 

 variation which the permeability of the iron undergoes when 

 33 is changed to 33'. It is, however, essential to distinguish 

 between fi and /j, if the plan is followed of finding a; by com- 

 paring the inductions reached in the two cases under equal 

 values of the magnetomotive force. 



A better way, however, is to draw the curve of magneti- 

 zation in relation to magnetomotive force for each of the two 

 cases, and then find, by measurement from the curves, what 

 is the difference in magnetomotive force for one and the 

 same value of the induction. This simplifies matters by 

 securing that ii shall be the same when there is a joint as 

 when there is none. If we write § for the magnetic force 

 when there is no joint, and «£)' for the mean magnetic force 

 required to produce the same induction 33 when there is a 

 joint, we have 



ft/ ® l 



and 



from wdiich 





x 



-<&} 



Thus, let a curve giving 33 and 1q be drawn, and another 

 curve, with the same axes, giving 33 and «£)', then the distance 



