COMMUTATION 143 



that the orthodox method of introducing self-induction and 

 mutual induction as separate entities endowed with certain prop- 

 erties peculiarly their own is not without advantages in the 

 solution of many problems, especially when mathematical analysis 

 is resorted to, but it tends to obscure the issue when seeking a 

 clear understanding of the physical aspects of commutation. 

 The splitting up of the magnetic induction resulting from dif- 

 ferent causes into several components is frequently convenient 

 and should not be condemned except in certain cases when iron 

 is present in the magnetic circuit. It cannot, however, be de- 

 nied that self-induction and mutual induction are frequently 

 thought of as different from other kinds of induction. We are 

 indebted for this state of. things to some writers whose familiarity 

 with mathematical methods renders a clear physical conception 

 of complicated phenomena unnecessary, but the practical engi- 

 neer or designer who produces the best work, especially in de- 

 partures from standard practice, is usually he who has the clearest 

 vision of the physical facts involved in the problem under con- 

 sideration. If the term self-induction calls up a mental picture 

 of magnetic lines, being a certain component expressed in 

 maxwells of the total or resultant flux of induction in a circuit, 

 this does not prevent our speaking of flux linkage per ampere 

 of current as inductance expressed in henry s and using the 



di 

 formula e = L -r. to calculate that component of the total e.m.f. 



in a circuit which would have a real existence if the field due to 

 the current i in the wire were alone to be considered. 



Following the lead of MR. LAMME, the wires in the coil under- 

 going commutation will be thought of as cutting through a total 

 flux of induction, expressed in magnetic lines or maxwells, this 

 flux being the result of the magnetizing forces of field coils and 

 armature windings combined. 



In Fig. 54 the thick-line rectangle represents a full-pitch 

 armature coil of T turns undergoing commutation. The dotted 

 rectangles show the position of two consecutive field poles, and 

 the shaded curve represents the ascertained or calculated flux 

 distribution over the armature surface. The ordinates of this 

 curve indicate at any point on the periphery the density of the 

 flux entering the armature core. The direction of slope of 

 the shading lines indicates whether the flux is positive or 

 negative. A method that may be followed in predetermining 



