Momentum in the Electric Field. 339 
and therefore no momentum; in the region between the 
spheres the magnetic force at a point P has its components 
Fig. 2. 
a, 8, y given by the equations 
Laweuy __ ewe ne? 
aoe 3.0re Y—°: 
the components f, g, 4 of the dielectric polarization, which 
are required for calculating the momentum, may for this 
purpose, as we are neglecting terms involving w’, be taken 
as unaltered by the motion and given by the equations 
i a 
Pears 4d OP* 77 ie OFF 
Hence, calculating the momentum in the shell by the 
ordinary rule, we find that it.is parallel to z and equal to 
2 ew 
3 OB’ 
where OB is the radius of the inner surface of the shell. 
Inside the inner shell the steady field has been established, 
so that if O’ is the position of O at the time ¢, the magnetic 
force at P’ will have for components 
< 
sew ips B=—ew orp Y=0. 
As we are neglecting squares of w and OO'=w?, we may in 
ealculating the momentum suppose that O! coincides with O: 
hence by the result given on p. 337, the momentum inside the 
sphere is parallel to z and equal to 
Sn {+ 1 
3 5 OB}? 
