CHAPTER IV. 

 MAGNETIC EFFECT OF THE ELECTRIC CURRENT.* 



50. The magnetic field due to an electric wire. The behavior 

 of a compass needle in the neighborhood of an electric wire shows 

 that the region surrounding an electric wire is a magnetic field. 

 The lines of force of this magnetic field encircle the wire. Thus 

 Fig. 57 shows the way in which iron filings arrange themselves 



Fig. 57. 



in filaments around a long straight electric wire, the black circle 

 at the center of the figure represents a section of the wire, which 

 is perpendicular to the plane of the figure. 



The lines of force of the magnetic field due to a circular loop 

 or coil of wire are shown in Fig. 58. In general, the lines of 

 force of the magnetic field produced by a coil of wire trend in- 

 wards toward the opening of the coil at one end, pass through 

 the opening of the coil, and spread out at the other end. Thus, 



* Chapter V on Induced Electromotive Force, and Chapter VI on Inductance con- 

 stitute continuations of this general subject, the Magnetic Effect of the Electric 

 Current. 



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