304 
cause of the magnetic field, in this case pro- 
duced by the rotation of the charges. There 
would be a similar result, and a similar inter- 
pretation, if the rod alone were to be given the 
charge and rotated. 
Again, if we take a metal rod and rotate it in 
a magnetic field, electric currents will in gen- 
eral be induced in it; and the magnetic field 
due to these currents will, if the rod is made 
of magnetic material, change its magnetiza- 
tion. Experiments of this kind were made 
about one hundred years ago by Barlow, 
Christie and Arago. 
In each of these cases, and in others which 
might be mentioned, a magnetic field is pro- 
duced by the rotation, and it is this field which 
produces the magnetization if a magnetic 
substance is present. a 
Coming now to the other or gyroscopic 
process of magnetization, and starting with a 
neutral rod of iron or other magnetic sub- 
stance, we can magnetize it directly by mere 
rotation, and a magnetic field will result from 
this magnetization. 
In order to understand this process it is 
necessary to consider first, a simple case of 
the behavior of a gyroscope; and second, the 
modern interpretation of Ampére’s theory of 
molecular currents. 
Here we have a gyroscope whose wheel, 
pivoted in a light frame, can be rotated 
rapidly about its axis A. Except for the ac- 
tion of two springs, this frame and the axis 
A are free to move in altitude about a hori- 
zontal axis B, perpendicular to A; and the 
axis B and the whole instrument can be ro- 
tated about a vertical axis C. If the wheel is 
spun about the axis A, and the instrument 
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 di- 
rection of the impressed rotation about the 
vertical axis C. 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 according to the modern version of 
SCIENCE 
[N. 8. Vou. XLVIII. No. 1237 
Ampére’s hypothesis, each molecule of a mag- 
netic substance has a magnetic moment, or is 
a magnet, because it consists in part at least 
of electrons revolving in fixed orbits with con- 
stant angular velocities about an oppositely 
charged nucleus, and producing a minute mag- 
netic field somewhat like that due to a small 
loop of wire traversed by an electric current. 
Tf these electrons, revolving in the same gen- : 
eral direction, have mass, each molecule has 
therefore angular momentum like the wheel 
of a gyroscope; and if the body of which it is 
a part is set into rotation about any axis, the 
molecule must change its orientation in such 
a way as to make the direction of revolution 
of its electrons coincide more nearly with the 
direction of the impressed rotation. 
Only a slight change of orientation can 
occur on account of the forces due to adjacent 
molecules, which perform the function of the 
springs in the experiment with the gyroscope. 
The rotation thus causes each molecule to 
contribute a minute angular momentum, and 
thus also a minute magnetic moment, parallel 
to the axis of rotation; and thus the body, 
whose molecular magnets originally pointed in 
all directions equally, becomes magnetized. 
If the revolving electrons are all positive, 
the body will become magnetized in the direc- 
tion in which it would be magnetized by an 
electric current flowing around it in the direc- 
tion of the angular velocity imparted to it. 
“Tf they are all negative, or if the effect of the 
negative electrons is preponderant, it will be 
magnetized in the opposite direction. This is 
what actually happens. 
For a simple type of molecular magnet a 
somewhat exact theory of the effect can be de- 
veloped. 
Assume:the molecule (Fig. 1) to consist of 
m (one or more) similar electrons, all positive 
or all negative, with total charge ne and total 
mass nm, revolving in a circular orbit of 
radius r with constant angular velocity » (and 
areal velocity a—4wr?) about a much more 
massive, and fixed, nucleus with charge —vne. 
This molecule will have a magnetic moment 
#=nea, a moment of inertia about the axis of 
revolution C—=nmr’, and an angular momen- 
