228 BELL SYSTEM TECHNICAL JOURNAL 



strength is applied to such a substance, you might expect all the little 

 magnets to turn right around and point straight up the field-direction, 

 achieving saturation in an instant. Well, it is certain that saturation 

 is not achieved; but still there is some degree of magnetization, as 

 though the little magnets all started to turn around and were stopped 

 before they got very far. What is it that might stop them? If you 

 look at the books of twenty or twenty-five years, you will find an 

 answer: they are stopped by the collisions which these atoms make with 

 one another. This is the classical idea, which is generally thought to 

 be well verified by experiment. But let us look into the matter a little 

 more closely. 



For simplicity let us imagine a gas — preferably, unit volume of the 

 gas — composed of N identical atoms, each with a magnetic moment n, 

 and exposed to an applied field H. Visualize some particular atom, of 

 which the career is an endless alternation between free flights and 

 sudden impacts. All the time the magnetic moment of the atom, the 

 little vector of which I have been speaking, is subject to a torque arising 

 from the field. The classical idea is, that throughout every free flight 

 that torque is steadily bringing the vector more and more nearly into 

 alignment with the field, but usually not having time enough to suc- 

 ceed, because at every collision the vector is suddenly and violently re- 

 directed in a perfectly arbitrary way. Gradual approach to alignment 

 during the free flights, violent dis-alignment at the collisions, and the 

 magnetization of the substance indicating how far the alignment 

 progresses, on the average, before the dis-alignment stops it — this is the 

 classical picture. It all seems beautifully obvious, and yet is it now 

 believed to be entirely false! 



The trouble lies in the fact that the atom is a gyroscope. You recall 

 that it is one of the oddities of the gyroscope that when you apply a 

 torque to it, it starts off at right angles to the direction in which you 

 expect it to go. Now here is our atom just leaving the scene of a colli- 

 sion with its magnetic moment making, say, an angle (f> with the field- 

 direction. As it flies away the torque is steadily trying to reduce the 

 value of 0, but instead of obeying, the atom just keeps on blandly pre- 

 cessing about the field-direction, the value of <^ remaining obstinately 

 the same. The unbreakable link between magnetic moment and 

 angular momentum has neatly killed the supposition that the field 

 magnetizes the gas because it aligns the atoms, or partially aligns them, 

 during their intervals of unimpeded flight. The free flights are just 

 the periods when nothing whatever happens in the way of alignment. 

 Much labor has been expended in the hope of finding some way out of 

 this impasse, but none has been revealed except that of supposing that 



