Photo-chemistry / by S.E. Sheppard.

  • Samuel Edward Sheppard
Date:
1914
Catalogue details

Licence: In copyright

Credit: Photo-chemistry / by S.E. Sheppard. Source: Wellcome Collection.

    76/484 page 60
    A casual inspection reveals certain interesting properties. The curves never intersect, but each lies above one for a lower temperature, i.e. the energy of every wave-length increases with increased temperature. Each curve has a maximum, the energy diminishing on either side. The wave-length at which this maximum lies is termed A.max, the corresponding maximal emissivity Smax- Further, it will be noticed that the position of Amax lies for different curves in different parts of the spectrum, and, in fact, with rising temperature, the maximum is con- tinually shifting toward the shorter wave-lengths, so that with increasing temperature the energy of the shorter wave-lengths increases more rapidly than that of the longer; thus an incandescent body first glows red, then passes to white heat. § 26. The Displacement Law. W. Wien, in 1893,1 by an application of Doppler’s principle,- obtained the law for this displacement, previously deduced empirically in 1888 by Weber. Wien found the relation between colour and temperature involved in the shortening of wave-length to be— x _r X’ ~ T that is, when the temperature increases, the wave-length of every monochromatic action radiation diminishes in such a manner that the product of temperature and wave-length is constant: Weber’s maximum relation Anmx T = constant appears as a special case. If this relation be combined with the Stefan law, we obtain for the displacements the two relations- Kax T = A = 2940 and Snmx = BT “ The maximal energy is proportional to the fifth power of 1 Wied. Ann., 58, 662 (1896). 2 Change in vibration frequency observed when either the source or the observer is moving in the line of transmission. See E. C. C. Baly, Spectroscopy, p. 535.
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