Page:Popular Science Monthly Volume 63.djvu/446

442 ground, there is a propagation through the earth of electric action, which may consist in a motion or atomic exchange of electrons. Each change or movement of a semiloop of electric strain above ground has its equivalent below ground in inter-atomic exchanges or movements of the electrons, on which the ends of these semi-loops of electric strain terminate. The earth must play therefore a very important part in so-called 'wireless telegraphy,' and we might almost say the earth does as much as the ether in its production.

The function of the receiving aerial is to bring about a union between these two operations above and below ground. When the electric waves fall upon it, they give rise to electromotive force in the receiving aerial, and therefore produce oscillations in it which, in fact, are electric currents flowing into and out of the receiving aerial. We may say that the transmitting aerial, the receiving aerial and the earth form one gigantic Hertz oscillator. In one part of this system, electric oscillations of a certain period are set up by the discharge of a condenser and are propagated to the other part. In the earth, there is a propagation of electric oscillations; in the space above and between the aerials, there is a propagation of electric waves. The receiving aerial feels therefore what is happening at the distant aerial and can be made to record it.

We have next to consider the question of the wave detecting devices which enable us to appreciate and record the impact of a wave or wave train against the aerial. At the very outset it will be necessary to coin a new word to apply generally to these appliances. Most readers are probably familiar with the term 'coherer,' which was applied by Sir Oliver Lodge, in the first instance, to an electric wave-detecting device of one particular kind, viz., that in which a metal point was lightly pressed against another metal surface and caused to stick to it when an electric wave fell upon it. As our knowledge increased, it was found that there were many cases in which the effect of the electric radiation was to cause a severance and not a coherence, and hence such clumsy phrases as 'anticoherer' and 'self-decohering coherer' have come into use. Moreover, we have now many kinds of electric wave detectors based on quite different physical principles. At the risk of incurring reprobation for adding to scientific nomenclature, the author ventures to think that the time has arrived when a simple and inclusive term will be found useful to describe all the devices, whatever their nature, which are employed for detecting the presence of an electric wave. For this purpose the term kumascope, from the Greek xνμα (a wave), is suggested. The scientific study of waves has already been called