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polarization and magnetization are intimately interwoven. Indeed, it 
is not always easy to separate them. 
We shall assume that our system of reference is such that ga 
vanishes for a— 1, 2,3. This means, that in our system of reference 
the velocity of beams of light in opposite directions is the same. 
This also implies that whenever the first three contravariant 
components of velocity vanish, also the first three covariant components 
vanish. 
Then we may in stationary points of matter (or in an arbitrary 
‘point after changing variables in a way that renders it stationary) 
separate 7’*) into two tensors, one consisting of the (a4)-components, 
the other components vanishing, and vice versa. These teusors we 
may call the polarization and magnetization tensors. It should be 
understood, that a change of variables, or a motion of matter, cannot 
leave half of the components zero, as they are in stationary points: 
the magnetization tensor, e.g., completes itself with polarization 
components. 
Write „Tet for 7 in a stationary point, and separate: 
„Tab = ,Mab 4 „Pb, 
so that 
Py bi ne 0 
‚Met (=) RT aps Lou 
nf lee elt 
0 0 0 
and 
0 0 agree 
»P (=) yO Ont IE 
0 0 „Ds 
pe Aad wapen 
We could have taken the covariant components ot the tensor 
T,, and the separation would have had the same effect. This is 
due to the fact that g,4 vanishes for a= 1, 2,3. If this is not the 
case, then we have first to change variables to make them vanish 
and afterwards make the separation. 
15.2. We shall now briefly indicate what becomes of the constitutive 
relations between polarization and electric force, magnetization and 
magnetic force, and between the conduction current and electric 
foree. We proceed in a quite formal way 
First, to find the generalization of the equation P = (e—1) EB, we 
form from the field-tensor a force-vector 4%: 
57* 
