Page:Popular Science Monthly Volume 63.djvu/449

Rh In the discussion which followed the reading of this paper, Professor Minchin described the effects of electric radiation on his impulsion cells. He followed up this by reading a paper to the Physical Society on November 24, 1893, on the action of Hertzian radiation on films containing metallic powders, and expressed the opinion that the change in resistance of the Branly tube was due to electric radiation.

Thus, at the end of 1893, a few physicists clearly recognized that a new means had been given to us for detecting those invisible ether waves, the chief properties of which Hertz had unraveled with surpassing skill six years before, by means of a detecter consisting of a ring of wire having a small spark gap in it.

In June, 1894, Sir Oliver Lodge delivered a discourse at the Royal Institution, entitled 'The Work of Hertz,' and at this lecture use was made of the Branly tube as a Hertz wave detecter. The chief object of the lecture was to describe the properties of Hertzian waves and their reflection, absorption and transmission, and many brilliant quasioptical experiments were exhibited. Although a Branly tube, or imperfect metallic contact, then named by him a coherer, was employed by Sir Oliver Lodge to detect an electric wave generated in another room, there was no mention in this lecture of any use of the instrument for telegraphic purposes.

As we are here concerned only with the applications in telegraphy, we shall not spend any more time discussing the purely scientific work done with laboratory forms of this wave detector.

Without attempting to touch the very delicate question as to the precise point at which laboratory research passed into technical application, we shall briefly describe the forms of kumascope which have been devised with special reference to Hertzian wave telegraphic work. A very exact classification is at present impossible, but we may say that telegraphic kumascopes may be roughly divided into six classes. The first class includes all those that depend for their action on the 'coherer principle' or the reduction of the resistance of a metallic microphone by the action of electromotive force. As they depend upon an imperfect contact, they may be called contact kumascopes. This class is furthermore subdivided into the self-restoring and the non-self-restoring varieties. The second class comprises the magnetic kumascopes