where under we have to understand the vector

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Now, we pass to the discussion of special theories, where we confine ourselves to isotropic bodies throughout.

§ 6. **Theory of H. Hertz.**

Hertz's electrodynamics of moving bodies sets the vectors and proportional to and

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Accordingly,

where and are considered as constants for a certain material point of a moving body.

Thus it follows from (18)

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*Hertz's theory doesn't know the concept of electromagnetic momentum*. It derives the ponderomotive force from the stresses alone, where it is irrelevant according to (10), whether one relates the stresses to stationary or to co-moving surfaces. A torque of the relative stresses doesn't arise, and also both sides of (18a) are equal to zero.

The energy density has a value according to (19)

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The simple approach, by which Hertz's theory connects the excitations with the electromagnetic forces , was, however, not confirmed by experiment as mentioned above. Thus only the choice between the theories to be discussed in the following paragraphs, remain.

§ 7. **Theory of E. Cohn.**

E. Cohn based the electrodynamics of moving bodies on the following connecting equations

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