THE MAGNETIC CIRCUIT 



[ART. 1 



the simplest magnetic circuit, because the field is uniform ami is 

 entirely confined within the winding. 



Iron filings orient themselves within the coil in directions indi- 

 cated in Fig. 1 by the concentric lines with arrow-heads. These 

 lines show that the medium is " magnetized " along circles con- 

 cent rie with the ring. Lines which show the directions in which a 

 medium is magnetized are generally called magnetic lines of force. 1 

 They are analogous to the lines of electrostatic displacement, 

 though their directions and physical nature are entirely different; 



SECTION A-A 



FIG. 1. A simple magnetic circuit. 



see the chapter on the electrostatic circuit, in the author's 

 Electric Circuit. 



The positive direction of the lines of force is purely conven- 

 tional, and is defined as that in which the north-seeking end of a 

 compass moves. Its relation to the current is, by experiment, that 

 given by the right-hand screw rule. Namely, if the direction of 

 the flow of a current is that of the rotation of a right-hand screw, 

 the lines of force point in the direction of the progressive move- 

 ment of the screw. Reversing the current reverses the direction 



1 For actual photographs, showing iron filings which map out the magnetic 

 field inside of coils of various shapes, see Dr. Benischke, Die Wissenschaft- 

 lichen Grundlagen der Elektrotechnik (1907), p. 126; also his Transformatoren 

 (1909), pp. 4, 6, and 57. 



