Popular lectures and addresses / by William Thomson.

  • William Thomson, 1st Baron Kelvin
Date:
1889-
Catalogue details

Licence: Public Domain Mark

Credit: Popular lectures and addresses / by William Thomson. Source: Wellcome Collection.

    173/486 page 155
    spectrum. The lowest, marked by Fraunhofer with the letter A, has for wave-length 7i/ioo,oooths of a centimetre. On the model before you I will now show you what is meant by a wave-length ; it is not length along the crest, such as we some- times see well marked in a wave of the sea break- ing on a long straight beach; it is distance from crest to crest of the waves. [This was illustrated by a large number of horizontal rods of wood connected together and suspended bifilarly by two threads in the centre hanging from the ceil- ing ;1 on moving the lowermost rod, a wave was propagated up the series.] Imagine the ends of those rods to represent particles. The rods them- selves let us suppose to be invisible, and merely their ends visible, to represent the particles acting upon one another mutually with elastic force, as if of indiarubber bands, or steel spiral springs, or jelly, or elastic material of some kind. They do act on one another in this model through the 1 The details of this bifilar suspension need not be minutely described, as the new form, with a single steel pianoforte wire to give the required mutual forces, described below and represented in Fig. 34, is better and is more easily made.
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