THE MAGNETISM OF IRON. 8 1 



Dividing both members of this equation by the sectional area of 

 the region between poles in Figs. 45 and 46, we get the force per 

 unit area which is transmitted across the region, or in other words, 

 the tension of the magnetic field. Therefore 



Tension of a magnetic field in 1 H 

 dynes per square centimeter f g 77- 



in which H is the intensity of the field in gausses. 



Energy of the magnetic field. If the magnet poles in Fig. 45 

 or 46 are allowed to move together, their force of attraction will 

 do an amount of work, W=Fd t where d is the initial distance 

 apart of the two pole faces, and the mechanical work thus gained 

 comes from the magnetic field that existed in the air space. 

 Therefore, using the value of F from equation (ii) we have 



W =Fd=H 2 s 



but sd is the volume of the region between the poles, so that 



Energy of a magnetic field in 1 H 



ergs per cubic centimeter j 



\ // 



45. The magnetization of iron.* When a piece of iron or other 

 magnetic substance, such as cobalt or nickel, is placed in a mag- 

 netic field, it becomes a magnet. For example, a neutral or 

 unmagnetized bar of iron or steel when held in the direction of 

 the earth's magnetic field shows north polarity at one end and 

 south polarity at the other end (the polarity of the bar may be 

 indicated by a compass needle). If the bar is turned end for end 

 its magnetism is reversed. A sharp blow with a hammer renders 

 the bar more susceptible to the influence of the weak magnetic 

 field of the earth. This action of a magnetic field upon iron is 

 called magnetization. 



When a piece of iron is placed in a magnetic field the trend of 



* For a full discussion of the theory of the magnetization of iron the student is re- 

 ferred to Franklin and Esty's Elements of Electrical Engineering, Vol. I, Appendix 

 A ; to J. A. Ewing's Magnetic Induction in Iron and Other Metals, London, 1900 ; 

 and to H. DuBois' Magnetic Circuit in Theory and Practice, translated by Atkinson, 

 New York, 1896. 



