250 METHODS OF CALCULATION 



of the field magnets, properly so called. For this purpose it suffices 

 to seek upon the characteristic of the armature the corresponding 

 E.M.F. db=e; the ordinate Qb corresponding to this abscissa e upon 

 the curve of the armature measures the necessary ampere-turns to 

 be produced between the poles, the necessary stray-fields /2 included. 

 We add thereto the ampere-turns equilibrating the direct reaction of 

 the armature, that is to say, 



KN 2 I d 



bq = 



V~2 



The ordinate Qq represents the difference of potential (subject to the 

 factor 0.47:) necessary between the poles of the field magnet. From 

 this may be deduced the value of the stray field/! between the pole 

 pieces, which may be presented, for example, as a function of the 

 magnetic difference of potential along the curve XP, which is sensibly 

 a straight line; Pp will then represent the stray field/!. 



If, starting from b, a segment bC be drawn representing Pp 

 (measured to the same scale as the flux db corresponding to the 

 E.M.F. e}, and through C we draw the straight line Cm parallel to 

 OP, the latter will contain between the two characteristics a segment 

 mn which will represent the fall of magnetic potential in the field 

 magnets under the influence of the total flux dC. The total 

 necessary M.M.F. will thus be equal to OP+mn. 



The diagram is thus established, taking into account the stray 

 field both of the field magnet and of the armature. It is distinguished 

 from those of Potier, 1 Rothert, 2 and Bauch, 3 because it takes into 

 account the transverse reaction with its real value; it takes account 

 of the difference between the two coefficients of reaction K and K t 

 and is thus distinguished from the diagram of M. Guilbert 4 for unsat- 

 urated field magnets; it finally differs in diagrammatic construction 

 from the very ingenious diagram of the same author for saturated 

 field magnets in the fact that it does not separate the air-gap from 

 the armature, and is also much more simple. 



Summing up, the employment of the diagram in Fig. 4 is to be 

 recommended for the case in which there is no appreciable saturation 

 either in the armature or in the field. In all other cases it seems 



i Potier, Eclairage Electrique, July 26, 1902. 



* A. Rothert, Elek. Zeit., 1899. 



3 Bauch, Elek. Zeit., 1902. 



< M. Guilbert, Revue Technique, April and May, 1904. 



