14 



PRINCIPLES OF ELECTRICAL DESIGN 



dimensions in centimeters; no constants have then to be in- 

 troduced because, with the C.G.S. system of units, 



Flux in maxwells = 



m.m.f. in gilberts 

 reluctance in oersteds 



The sketch, Fig. 1, shows a (closed) tube of induction linked 

 with a coil of wire of S turns through which a current of / amperes 

 is supposed to be passing. This tube of induction consists of a 

 number of unit tubes or so-called magnetic lines each of which is 

 closed on itself. It follows that the total flux 3> is the same 

 through all cross-sections of the magnetic tube of flux indicated 



FIG. 1. Tube of induction linked with coil. 



in Fig. 1. The cross-section may, and generally does, vary from 

 point to point of the magnetic circuit, and since the total flux 

 $ is of constant value, the density B will be in versely proportional 

 to the cross-section. Thus, at a given point where the cross- 

 section is A i square centimeters the density in gausses is B\ = 



Turning again to the fundamental formula of the magnetic 

 circuit, we have, 



m.m.f. = flux X reluctance 

 or 



gilberts = maxwells X oersteds 

 or 



OAwSI = $ X (-r- + -A- + etc.) (3) 



\AiUi AZU.Z I 



Also, since m.m.f. = magnetizing force X length of path, it is 

 sometimes convenient to put the above general expression in 

 the form 



0.47nS7 = #1*1 -f #2*2 + etc. (4) 



