Mat 20, 1921] 



SCIENCE 



473 



sumed; and whicli has therefore the dynam- 

 ical properties of a gyroscope. It will now be 

 shown how these gyroscopic properties have 

 made possible a complete and direct demon- 

 stration of the correctness of this theory. 



-^—\c 



Fig. 8. 



In Fig. 8 is shown a gyroscope whose wheel, 

 pivoted in a frame, can be rotated rapidly 

 about its axis A. Except for the action of 

 two springs, the frame and the axis A are 

 free to move in altitude about a horizontal 

 axis B, making an angle $ with the vertical C ; 

 and the axis B and the whole instrtiment can 

 be rotated about the vertical axis C. If the 

 wheel is spun about the axis A, and the in- 

 strument then rotated about the vertical C, 

 the wheel tips up or down so as to make the 

 direction of its rotation coincide more nearly 

 with the direction of the impressed rotation 

 about the vertical axis 0. If it were not for 

 the springs, the wheel would tip until the axes 

 A and C became coincident. The greater the 

 rotary speed about the vertical the greater is 

 the tip of the wheel. When the wheel's speed 

 about the axis A is zero, no tip occurs. 



Now if the magnetic molecule is a gyro- 

 scope, it will behave like this wheel. If the 

 body of which it is a part is set into rotation 

 about any axis, the molecule, or magneton, 

 will change its orientation in such a way as to 

 make its direction of rotation coincide more 

 nearly with the direction of the impressed 

 rotation; the coincidence will finally become 

 exact if this is not prevented by the action of 

 the rest of the body. This idea was in the 



mind of Maxwell in 1861, and has occurred to 

 a number of others since. 



In an ordinary ferromagnetic body in the 

 usual state with which we are familiar only 

 a slight change of orientation can occur on 

 accunt of the forces due to adjacent molecules, 

 which perform the function of the springs in 

 the case of our gyroscope. The rotation 

 causes each molecule to contribute a minute 

 angular momentum, and thus also a minute 

 magnetic moment, parallel to the axis of im- 

 pressed rotation; and thus the body, whose 

 magnetons originally jwinted in all directions 

 equally, becomes magnetized along the axis 

 of impressed rotation. If the revolving elec- 

 trons, or rotating magnetons, are all positive, 

 the body will thus become magnetized in the 

 direction in which it would be magnetized by 

 an electric current flowing around it in the 

 direction of the angular velocity' imparted to 

 it. If they are all negative, or if the action 

 on the negative magnetons is prejwnderant, 

 it will be magnetized in the opposite direction. 



If B denotes the ratio of the angular 

 momentum of a magneton, or an electron 

 orbit, to its magnetic moment, it can readily 

 be shown that rotating a magneton or electron 

 orbit about any axis with an impressed 

 velocity N revolutions per second, is equiva- 

 lent to placing it in a magnetic field with 

 intensity H directed along this axis such that 

 H = R27rN. 



If the electric density is proportional to 

 the mass density throughout the volume of 

 the magneton this ratio is easily shown to be 

 R = 2m/e ; so that in this case 



H='ln-N. 



(16) 



If all the magnetons in a body are alike, 

 therefore, rotating it at an angular velocity 

 of N r.p.s. will produce the same intensity of 

 magnetization in it as placing it in a field of 

 strength 47r(m/e)iV gausses. 



For negative electrons of the ordinary type 

 47r(m/e) ^ — Y.l X 10-' e.m.u. according to 

 well-known experiments. Hence, if orbital 

 motions of these electrons are responsible for 

 the magnetism of ferromagnetic substances, 

 rotating them at a velocity of N revolutions 



