All quantities in this chapter are expressed in "electromag- 

 netic" c.g.s. units: magnetic field intensity in gausses, magnetic 

 flux in lines (maxwells), current in abamperes, length in centi- 

 meters, area in square centimeters, etc. 



CHAPTER IV. 



ELECTROMAGNETS. MAGNETISM OF IRON. 



45. Bunched windings and distributed windings. The electro- 

 magnet, such as is used for the field magnet of a dynamo, consists 

 of a rod of iron surrounded by a winding of insulated copper 

 wire, through which an electric current flows. The iron rod is 

 called the core of the electromagnet, and the electric current 

 which flows through the winding of wire is called the exciting 

 current. The exciting current produces a magnetic field in the 

 region occupied by the iron core, and the effect of this magnetizing 

 field, as it is called, is to magnetize the core and cause a certain 

 amount of magnetic flux to "flow" through it. The iron rod or 

 core usually forms a complete or nearly complete circuit, called 

 a magnetic circuit, through which the magnetic flux "flows." 



Two distinct cases occur in the arrangement of the windings 

 of wire upon the iron core as follows : 



(a) Uniformly distributed winding. The winding of wire may 

 be distributed uniformly along the entire length of the iron rod 

 or core. In this case the magnetic field which the winding 

 produces in the region occupied by the iron core has sensibly the 

 same value at every point in the rod and is sensibly parallel to 

 the rod at each point. The magnetizing action of such a field 

 upon an iron rod depends simply upon the intensity of the field. 

 Electromagnets having uniformly distributed windings are seldom 

 used in practice except in the magnetic testing of iron, for which 

 purpose the iron to be tested is usually made into a test ring,* 

 which is wound uniformly with wire. 



* This applies to the ballistic method of testing iron, which is due to Rowland. 

 In Ewing's method the test piece of iron is in the form of a long slim rod. 



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