204 THE REALITIES OF MODERN SCIENCE 



nickel and cobalt this effect is very pronounced. In 

 most other substances it is practically negligible. 

 The substances in which there is any effect whatever 

 are called " paramagnetic/' while those like iron are 

 known as " magnetic. " 



The group of loops which we have described is a 

 model of a magnetic body. Since the loops are free 

 to turn, thus reducing the potential energies of the 

 systems they form with each other, and since the greatest 

 reduction would have been made when the electrons 

 were moving in parallel paths, we might expect that 

 they would do so and hence that in a magnetic sub- 

 stance the molecules would naturally have assumed 

 similar orientations. Such, however, is not the case. 

 The molecules have haphazard orientations, as may be 

 verified by placing the body near another similar body 

 and noting that there is no attraction or repulsion, as 

 would be the case if their molecular currents were 

 not flowing " every which way." The explanation 

 is that the molecules have already formed themselves 

 into a large number of small and fairly stable groups. 



When such a body is placed near a loop carrying a 

 current some of these groups are disrupted, new groups 

 being formed with more molecules oriented in the direc- 

 tion of minimum potential energy with respect to the 

 magnetizing current. Two phenomena are now possible 

 when the magnetizing coil is withdrawn or its current in- 

 terrupted. Either these newly formed molecular groups 

 may break down, due to their interactions with adjacent 

 groups, or many of them may be so stable as to persist 

 more or less permanently. In the latter case the mag- 

 netic body is said to be permanently magnetized and is 



