The Propagation of Radio Waves over the Earth

THE modified form of the ionic deflexion theory of wave propagation, discussed by Messrs. Nichols and Schelling (NATURE, March 7, vol. 115, p. 334), appears to have been suggested in England and the United States almost simultaneously. That the effect of the earth's magnetic field had to be taken into account in calculating the phase-velocity was pointed out in a paper on "Geophysical Influences on the Transmission of Wireless Waves" read at a joint discussion of the Physical Society of London and the Royal Meteorological Society in November 1924 and already published. The formula for the phase-velocity for transmission along the earth's magnetic field and the calculation of the critical frequencies were there given, and some consequences of these effects have been discussed in a paper communicated to the Cambridge Philosophical Society. Since the earth's magnetic field is strong enough to affect the phase velocity to a sufficient extent, all the rotatory and double refraction effects familiar in physical optics are appreciable. But probably the most interesting possibility is that the reciprocity relation between two wireless stations may not hold, for the extra forces on the moving electrons due to the earth's magnetic field are, to a certain extent, independent of the direction of propagation of the waves and thus produce relatively different effects for the two directions of transmission.