x oe a 
. Sal p 
210 RICHARD THRELFALL. 
separate curves might have been drawn, but I did not think it 
worth while to introduce a fresh sheet of curves. 
Iam not sure that a real reduction in air gap reluctance at 
about one diameter has not been smoothed out, but as the 
observations are marked on the curves every 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 ‘3 diameters. It is perhaps not 
too much to say that the reluctance increases very fast as the gap 
increases to one and a-half diameters, after which it remains 
nearly constant. 
15. I do not know whether a unit of reluctance has yet been 
adopted. It has been necessary for me to use one however. I take 
as unit reluctance, that reluctance through which unit magneto- 
motive force produces unit induction. By unit magnetomotive 
force I mean that magnetomotive force whose C.G.S. value is unity 
—i.e. that produced by 47 C.G.8. current turns. If the perme- 
ability of air be taken as unity, then one cubic centimeter 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 47. 
16. 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 oper- 
ate over air gaps. For instance, with a given 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 magnetomotive force is given. 
