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 detector. The chief object of the lecture was to describe the properties of Hertzian waves and their reflection, absorption and transmission, and many brilliant quasi-optical 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.[32] 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_ which depend upon the action of an electrical oscillation as a magnetising or demagnetising agency. The third class comprises the _electrolytic responders_, in which the action of electric oscillations either promotes or destroys the results of electrolysis. The fourth class consists of the _electrothermal detectors_, in which the power of an electrical oscillation as a high frequency electric current to heat a conductor is utilised. The fifth class comprises the _electromagnetic_ or _electro-dynamic_ instruments, which are virtually very sensitive alternating-current ammeters, adapted for immensely high frequency. The sixth class must be made to contain all those which cannot be well fitted at present into any of the others, such as the sensitive responder of Schäfer, the action of which is not very clearly made out.