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BELL SYSTEM TECHNICAL JOURNAL 



Experimental evidence of thie existence of these domains is supplied 

 by the so-called "Barkhausen effect" (Fig. 5). If a small portion of a 

 magnetization curve such as is shown in Fig. 5 could be magnified a 

 billion times, it would be seen to be made up of steps, each a sudden 

 change in magnetization as the field is increase^d, with no further 

 change until the field reaches a certain higher value. No known 

 apparatus can give such direct magnification, but these sudden jumps 

 can be detected by winding a coil around the specimen and connecting 

 its ends to an amplifier at the output of which is a pair of telephone 



Fig. 5 — Sudden changes in magnetization cause the Barkhausen effect. 



receivers. When the field is slowly increased, a series of clicks, or 

 "noise," is heard in the receivers; a more quantitative method shows 

 that the average click corresponds to the reversal of magnetization in 

 a region the size ^^ of a cube 0.001 inch on an edge, containing 10^* 

 atoms. Under favorable conditions this "Barkhausen noise" can be 

 heard without an amplifier, with the receivers connected directly to the 

 coil. 



It has been pointed out that the forces of exchange are opposed by 

 the disordering forces of temperature agitation. As a result, the satu- 

 ration value of magnetization decreases continuously as the temperature 

 is increased, until at the Curie point the ferromagnetism disappears. 

 Data for saturation at various temperatures are shown in Fig. 6, 

 plotted in such units that the saturation is unity at the absolute zero 

 of temperature, and the Curie point is unity on the temperature axis. 

 On such a plot it is found that the data for iron, cobalt, and nickel fall 

 close together. The lower curve is the theoretical one calculated 



