﻿100 Approximate Method of calculating Magnetic Forces. 



2 diameters apart. The proper reluctance for the iron and 

 air circuits was taken from the curve in finding the reluctance 

 of the air-gap. 



Except with the largest bar, there is no definite indication 

 of the reluctance depending on the induction-density. In 

 this case separate curves might have been drawn, but I did 

 not think it worth while to introduce a fresh sheet of curves. 



I am not sure that a real reduction in air-gap reluctance at 

 about *2 diameter has not been smoothed out ; but as the 

 observations are marked on the curves, any one will be able 

 to form his own opinion. 



It will be noticed that the curvature becomes very great 

 when the air-gap amounts to about 1*5 diameter. It is, 

 perhaps, not too much to say that the reluctance increases 

 very fast as the gap increases to *2 diameter, after which it 

 increases more slowly up to about 1 diameter, and then 

 remains roughly constant. 



§ 17. I do not know whether a unit of reluctance has yet 

 been adopted. It has been necessary for me to use one, 

 however, and I take as unit reluctance that reluctance 

 through which unit magnetomotive force produces unit 

 induction. By unit magnetomotive force I mean that 

 magnetomotive force whose C.Gr.S. value is unity — i. e., that 



produced by =— C.G.S. turns. If the permeability of air be 



taken as unity, then one cubic centimetre of air has unit 

 reluctance on this system. There are, of course, other ways 

 of defining unit reluctance ; but this is, I think, the only one 

 that gets rid of the 4tt and leads to an easily realizable 

 material standard. 



§ 18. The reluctance curves and traction curves are not 

 unlike each other in general form, and enable us to draw 

 some practically valuable conclusions as to the design of 

 magnets intended to operate over air-gaps. For instance, 

 with a given total induction the force at contact is inversely 

 as the area, but the traction curve shows that this principle 

 must not be pushed too far when we consider traction over 

 an air-gap. Thus, I am told (though I do not believe it) 

 that rock-drills will not work with a shorter stroke than five 

 inches ; the traction curve shows at once that for a given 

 induction (a case which does not practically occur in every 

 instance) it is possible to make the pole-pieces too small, if 

 we wish to get the maximum work done during the stroke. 

 This is independent of considerations arising when magneto- 

 motive force, and not induction, is given. 



