THE MAGNETIC CIRCUIT 



CHAPTER I 



THE FUNDAMENTAL RELATION BETWEEN FLUX 

 AND MAGNETOMOTIVE FORCE 



1. A Simple Magnetic Circuit. The only known cause of 

 magnetic phenomena is an electric current, or, more generally, 

 electricity in motion. 1 The fundamental relation between an 

 electric current and magnetism can be best studied with the simple 

 arrangement shown in Fig. 1. A coil, CC, of very thin wire is uni- 

 formly wound in one layer on a spool made in the shape of a circu- 

 lar ring (toroid) . The tubular space inside of the ring is filled wit li 

 some " non-magnetic " material, so called; for instance, air, wood, 

 When a direct current is sent through the coil, the space 

 inside the coil is found to be in a peculiar state, called the magnetic 



This magnetic state can be experimentally proved 1>\ 



various means, such as a compass needle, iron filings, etc. A 



region in which a magnetic state is manifested is called a magnetic 



fn /'/. Tims, in Fig. 1, the tubular space inside the coil is the mag- 



field excited by the current in the coil CC. 



magnetic field is found in the space outside the coil upon 

 exploring ii with a magnetic needle or with iron filings. For 

 reasons of symmetry, the field inside the ring is the same at all 

 th< cross-sections. Thus, a uniformly wound ring constitutes 



\\f-rner v. Siemens, Wiedemann't Annalen, Vol. 24. (1885), p. 04; Lar- 

 mor, Ether and Matter (1904), p. 108. The magnetism of a permanent magnet 

 is probably due to molecular current* produced by some orbital motion of 

 electrons within the atoms of iron. The older concepts of magnetic charges 

 MM-! free poles are dismissed in this book as inadequate and 



