242 METHODS OF CALCULATION 



are spread out more or less over the entire length of the principal 

 circuit up to its entrance into the armature turns. This fact is, how- 

 ever, unimportant, as has recently been shown by M. Guilbert (see 

 Eclairage Electrique, December, 1903). 



(3) The self -inductance of the armature is produced by the stray 

 fields / 2 , supposedly attributed to the effect of the armature. If we 

 call R/ 2 the reluctance of the circuit of the stray fields f 2 , and R a the 

 reluctance of the armature, the stray field produced by the armature 

 across itself, is expressed in practical units (N 2 being the number of 

 peripheral conductors per pair of poles), 



[ KN 2 I 2 \/2 sin <p 1 o.2TiKN 2 I 2 \/ 2 sin 



* 1 2(R a +R h ~r ~fy~ 



assuming that we can neglect R a with respect to Rf. t , and thus pro- 

 duce an E.M.F. 



kN 2 o}Xo.i7iKN 2 I d 







2V 2 R h 



It results from this that the E.M.F. of self-induction that we have 

 called ojsld can be considered as produced simply by a stray field 

 /2, which is added to the stray field of the field magnets /i. 



Upon the saturation curve XM (Fig. 2) defined as above, the 

 point b which corresponds to the E.M.F. OB (Fig. i), represents the 

 NI necessary to force the useful flux through the field magnets into 

 the armature. Adding to the flux <# a the stray field of the armature 

 /2, there is obtained the virtual E.M.F. OD=e, corresponding to 

 the total flux emanating from the poles into the air-gap; the corre- 

 sponding abscissa XQ' represents the necessary field-winding ampere- 

 turns NI!I', without taking into account the increase J/j of the 

 stray field /i of the field magnet. 



(4) The stray field of the field magnets f\ is inversely propor- 

 tional to the reluctance of the stray path /?/, between the poles and 

 directly to the difference of magnetic potential between the poles. 

 This latter is formed of two parts; one part is the drop of mag- 

 netic potential necessary to force the flux through the armature and 

 air-gap, the other part, the reactive counter-ampere-turns of the 

 armature calculated as above. 



(5) Every increase in the ampere-turns of the field magnet increases 



