Page:Zur Thermodynamik bewegter Systeme (Fortsetzung).djvu/4

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9. Application to cavity radiation.

We base our calculation on the relative ray path. We consider radiation that encloses angles between and with the direction of motion; it carries – in unit volume through the unit surface of a perpendicular (co-moving) plane – the energy amount:

We call the intensity of the total (relative) radiation. If this radiation is incident upon an absorbing surface, it performs the pressure work:[1]

where is the angle between the absolute radiation direction and the direction of motion. The difference:

we call the true (relative) radiation. The true radiation intensity


is crucial for the heat transport between bodies of equal velocity.[2]

We employ the standpoint of Lorentz's contraction hypothesis and introduce the angle by the equation

  1. M. Abraham, Boltzmann-Festschrift, p. 90, 1904. Compare for instance F. Hasenöhrl, Jahrb. d. Radioaktivität, 2, p. 281 (1905).
  2. This terminology agrees with the one used in an earlier paper (Ann. d. Phys., 15 [1904]). There, and was written instead of and . See also Jahrb. d. Radioaktivität und Elektronik, 2, p. 283 (1905).