The wave-length of Hertzian waves was found by different methods; it varies with the dimensions of the apparatus used for generating the waves. The length of the waves used in wire less telegraphy is measured in hundred's of metres, as against the few hundred-thousandths of a centimetre that give the length of visible light waves. Every source of alternating current is an emitter of very long electromagnetic waves : a dynamo, for instance, running at 3,00o revolutions per minute generates waves of length six thousand kilometres.
At the other end of the scale, in the region of very short waves, lie the X-rays, discovered by Röntgen in 1895, and the 7-rays, discovered by Villard in 1900. The nature of these rays was long in doubt, for until 1912 the view that they were neutral particles had its supporters, as well as the view that they were electromagnetic waves. In this year Laue, Friedrich and Knip ping established the diffraction of X-rays by crystals, and shortly after W. H. and W. L. Bragg showed how the wave-length of X-rays could be measured by reflection at crystal faces. (See X-RAYS, NATURE OF: Röntgen Rays; SPECTROSCOPY.) More re cently (1925, 1926) A. H. Compton and R. L. Doan, as well as J. Thibaud, have succeeded in demonstrating the diffraction of Rönt gen rays by ruled gratings, of the type used for measuring the wave-length of ordinary light, the rays being made to graze the grating at a very small angle. The refraction of Röntgen rays was established at about the same time, by making the rays strike the face of a glass prism, near the apex, at a very fine glancing angle.
The value found for the refractive index by these experiments, as also by the total internal reflection which can be established, is less than unity, as given by the theoretical dispersion formula. Although the velocity of the Röntgen rays has not been meas ured directly, all the other properties of an electromagnetic wave have been shown to be possessed by the Röntgen rays. In particular, the wave-lengths of all the characteristic X-rays have been carefully recorded, ranging from 22 A.U. to •i A.U. As for the 7-rays, in 1914 Rutherford and Andrade measured the wave length by the crystal method, and showed them to be of essen tially the same nature as Röntgen rays.
namely that comprising wave-lengths from 8X to 4 X io"' cm., but all the other radiations are propagated with the same velocity as visible light and exhibit the characteristic phenomena which are associated with transverse waves, in particular reflec tion, refraction, polarisation and interference in its many aspects. Recent researches have served to emphasize the common nature of radiations of widely different frequency : for instance E. V. Appleton has shown that interference phenomena can be obtained with the Hertzian waves used in wireless telegraphy by reflection from the Heaviside layer, and has repeated a variety of experi ments on interference and polarisation, usually performed in the laboratory with waves of length of the order 5X cm., over distances of miles with waves whose length is a few hundred metres. (See WIRELESS TELEGRAPHY.)' The whole electromagnetic spectrum is diagrammatically repre sented in the accompanying chart, in which the wave-lengths are set out vertically on a logarithmic scale, to avoid the compres sion to vanishing point of the shorter wave-length which would be inevitable if a linear scale were adopted. The ends of the spectrum are not definite : waves of frequency as low, that is, of wave-length as great, as may be desired can be generated by rotating a coil in a magnetic field, while at the short wave length end of the range we have the penetrating cosmic radia tions, whose wave-length is shorter than that of the 7-rays, but can only be roughly estimated from the absorption. Starting at the top with wave-lengths of .01 Angstrom units cm.), which are of the order of the shortest 7-ray measured by Ellis cm.), we proceed by way of longer 7-ray wave-lengths to X-rays, the rays used for radiotherapy lying in the region 6X cm. to io X cm. while those used in diagnosis lie generally between 2 and 3X cm. The X-rays merge into the ultra-violet, which in its turn leads into the visible spectrum. On the other side of the visible spectrum we have the infra-red, which leads to a vast range of Hertzian radiations, 28 octaves in extent, succeeded by the ill-defined region of very long waves to which reference has already been made. The frequencies, obtained by dividing the velocity 3X io'' cm./sec. by the wave-length in cms., are given on the extreme left : self-explanatory notes as to the method of generation and detection of the waves are added. De tailed information as to the various classes of radiation is given in the articles RADIOACTIVITY ; X-RAYS, NATURE OF : Röntgen Rays; SPECTROSCOPY ; LIGHT ; HEAT ; ELECTRIC WAVES ; WIRELESS TELEGRAPHY.