Displacement of Spectral Lines produced by Pressure. 575 



which could arise in this way are much too small to account 

 for the observed facts. 



Suppose the magnetic fields of the atoms are caused by the 

 motion of some or all of the constituent electrons in closed 

 orbits, which we may take as an approximation to be circular. 

 Although this hypothesis has been criticised by notable 

 authorities, it appears to the writer to have the balance of 

 probability in its favour. If n is the number of mobile electrons 

 in the atom, cr s the radius of the orbit of the sth electron, 

 and T 5 the time in which it is described, the atom will be 

 equivalent to a magnet of moment 



A = 7r-2ov/T electromagnetic units. . . (28) 



c i ' ° v y 



Here e is the charge on an electron in electrostatic units, 

 and c is the velocity of light. This is the maximum value 

 obtained on the supposition that the orbits are all parallel to 

 one another and the rotations all in the same sense. On 

 this view, the maximum intensity of magnetization which an 

 element is capable of is determined by the atomic amperean 

 currents. We shall assume that the magnetic field of any 

 atom is not greater than that which corresponds to saturated 

 iron, as iron is the most intensely magnetic substance of 

 which we have experience. This conclusion is supported by 

 the high value which the magnetic properties of iron indicate 

 for the number of rotating electrons in the atom. The 

 maximum intensity of magnetization I of soft iron is about 



- x 10 4 and I = vA, where v is the number of atoms of iron 



7T 



in 1 c.c. Eliminating A between this equation and equation 

 (28) we can get an estimate for n. Putting 1= - X 10 4 , 



v = l-4xl0 23 , o-=10- 8 , T=10- 15 , e/c = 10- 20 , we find n, the 

 number of rotating electrons in the atom, =7*3. As the 

 total number of electrons is probably somewhere near 56, 

 the value of the atomic weight, we see that in iron quite a 

 considerable fraction (one-eighth) of them are engaged in 

 producing the atomic magnetic field. It is therefore un- 

 likely that any element will possess an atomic magnetic field 

 greater than that calculated for iron from the magnetic pro- 

 perties of the metal in bulk. The value A = 2 x 10 " 20 electro- 

 magnetic units may probably be regarded as a superior limit 

 for an atom of any substance. 



The next step is to find how the magnetic field near the 

 atom A will be influenced by the presence of foreign atoms 



2Q2 



