APPENDIX A. 



ELECTROMAGNETS. MAGNETISM OF IRON. 



1. Bunched windings and distributed windings. The electro- 

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

 sists of a rod of iron surrounded by a winding of insulated 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 tends to produce 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 com- 

 plete circuit, called a magnetic circuit, through which the mag- 

 netic 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 

 tends to produce in the region occupied by the iron core has sen- 

 sibly the same value at every point in the rod and is sensibly par- 

 allel 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 Swing's method the test piece of iron is in the form of a long slim red. 



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