ELECTROMAGNETS. MAGNETISM OF IRON. 85 



field intensity in a narrow longitudinal* cavity is the actual 

 value of the magnetizing field in the iron. 



In the magnetic testing of iron, the iron sample is always a 

 very long slim rod or a closed ring so that the sample when 

 magnetized does not have any demagnetizing action upon itself. 

 That is to say, the magnetic field due to the magnetizing coil is 

 the net magnetizing field. 



Also, in transformers, the magnetic circuit is nearly all of iron 

 (without any free magnet poles), and therefore the magnetizing 

 action of the windings is not to any perceptible extent compli- 

 cated by demagnetizing action of the iron core upon itself. 



The above discussion (see Figs. 60, 61 and 62) is intended only 

 to lead to a clear understanding of the term net magnetizing field; 

 no attempt is made in this text to take account of demagnetizing 

 action in numerical calculations.! 



52. Intensity of magnetization. Let m be the strength of 

 each of the magnetic poles at the ends of a magnetized iron rod 

 of which the sectional area is 5 square centimeters. The pole- 

 strength-per-unit-sectional-area, namely m/s, is called the 

 intensity of magnetization, <?, of the rod. That is 



^=7 CD 



53. Magnetic flux through a magnetized rod. Consider a 

 very long slim iron rod, 5 square centimeters in sectional area, 

 placed in and parallel to a magnetic field, which, but for the pres- 

 ence of the rod, would be a uniform magnetic field of intensity 

 eft. Then the actual intensity of the magnetizing force along 

 the middle portions of the rod is cV, as explained in Art. 51. 

 The magnetic field in the neighborhood of each end of the rod 

 is a composite field which is the resultant of two distinct parts, 

 a and b\ and the magnetic flux which comes up to one end of 



* Parallel to the direction of magnetization of the iron. 



t Except in a very simple way in allowing for the effect of the air gaps in the 

 magnetic circuit of a dynamo. See Art. 57. 



