ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 83 
that the saturation in the unit section should correspond to 
the highest permeability. 
Further, it is necessary that the spaces between the useful 
paths of different magnetic density sheuld be large, but at 
the same time should offer as small a sectional area as 
possible to the leakage lines, and that the pole pieces 
should be placed far apart from each other. 
_ All these conditions cannot, however, be complied with, as 
it would involve a sacrifice of practically everything for 
_the sake of good regulation, so making a very costly 
alternator. 
The compromises which are to be effected in order to obtain 
an all round satisfactory machine, will be discussed later 
in the examples of design. 
140. Reluctance.—The symbolical expressions of the reluc- 
; tances of the different magnetic paths depending on the 
; dimensions and forms of the iron and copper parts, are 
given in Pars. 141 to 153. 
These items are of the greatest importance, because the 
proper determination of the machine characteristics de- 
pends on the correct calculation of the different magnetic 
resistances. 7 
With the careful determination of the no-load and short 
circuit characteristics, not only is the required regulation 
obtained within a specified limit, but expensive experi- 
ments and tests are unnecessary. 
141i. Calculation of Reluctance.—The magnetic resistances 
or reluctances of the various magnetic paths may be eal- 
eulated on the basis of Equation (18), by referring to 
Fig. 42, which indicates by symbols the dimensions of the 
various parts. 
It should be noted that the expressions of the reluctances, 
are obtained from hypothetical assumptions, simplified for 
practical use, so that they form more or less empirical 
formule of sufficient exactness for design purposes. 
