CHAPTER II 

 THE MAGNETIC CIRCUIT ELECTROMAGNETS 



In all dynamo-electric machinery, coils of wire carrying electric 

 currents produce a magnetic field in the surrounding medium 

 whether air or iron and the purport of this chapter is to show 

 how the magnetic condition due to an electric current can be 

 determined within a degree of accuracy generally sufficient for 

 practical purposes. 



Tbe design of the magnetic circuit of dynamo-electric genera- 

 tors does not differ appreciably from the design of electromagnets 

 for lifting or other purposes, and it is, therefore, proposed to 

 consider, in the first place, the fundamental principles and 

 calculations involved in proportioning and winding electro- 

 magnets to fulfil specified requirements. Particular attention 

 wffl be paid to types of magnets with small air gaps because these 

 serve to illustrate the conditions met with in field-magnet design, 

 and the principles of the magnetic circuit can be applied with 

 but little difficulty; while, in the case of coreless solenoids or 

 magnets with very large air gaps, the paths of the magnetic flux 

 cannot readily be predetermined, and empirical formulas or 

 approximate methods of calculation have to be used. When the 

 magnetic circuit is mainly through iron, and the air gaps are 

 comparatively short, it is generally possible to picture the lines 

 or tubes of magnetic flux linking with the electric circuit, thus 

 facilitating the quantitative calculation of the flux at various 

 parts in the circuit; but when the path of the magnetic lines 

 is largely through air or other "non-magnetic" material, the 

 analogy between the magnetic and electric circuits is less con- 

 venient and may indeed lead to confusion; the quantitative 

 calculations become more difficult and less scientific, calling for 

 an experienced designer if results of practical value are desired. 



1. The Magnetic Circuit Without dwelling on the mathe- 

 matical conceptions of the physicist, which may be studied in 

 all books on magnetism, it may be stated without hesitation 

 that the analogy between the magnetic and electric circuits, 

 and the idea of a closed magnetic circuit linked with every electric 



10 



