A DISCOVERY of great theoretical and practical significance in radio transmission was made last year. Listeners to foreign stations have, for some time past, noted that their reception was sometimes marred by a faint background of sound apparently made by a high-power long-wave station. At first, lack of selectivity in the receiver and possibly cross modulation were suspected. According to Prof. E. V. Appleton, in a paper in the Electrician of January 25, the interference is due to a cross modulation effect in one of the ionised layers in the atmosphere. The effect was first noticed in connexion with the powerful Luxembourg station, so it is generally called the Luxembourg effect. Recently amateurs belonging to the Radio Research League have shown that the phenomenon is also produced by the high-power stations at Droitwich and Athlone. Apparently a long-wave station of this type can impress the waves it produces on the ionised layer in its vicinity. If waves of another wave-length are reflected there, they acquire the modulation in question during the process of reflection. The present tendency of increasing the power of long-wave senders makes this phenomenon of practical importance. It brings about a type of interference over which the radio engineer has no control. Prof. Appleton comments also on the propagation of ultra-short waves (less than 8 metres). They are of importance because of their possible use in television. Apparently there is no acceptable evidence that ‘round the world’ communication will ever be possible with such short wavelengths. The lowest possible wave-length appears to be determined by the finite value of the electrification in the upper atmosphere.