HERTZIAN ^yAVE WIRELESS TELEGRAPHY. 451 



time which limits the speed of receiving. The tapper has to be so 

 arranged that it is possible to receive and to record not only the dot 

 but a dash on the Morse system. The dash is really a series of closely 

 adjacent dots, which run together in virtue of the inertia and induct- 

 ance of the dill'erent parts of the whole receiving apparatus. Tlic 

 adjustment has so to be made that, whilst the dash is being recorded 

 and a continuous tapping is kept up, yet, nevertheless, the continued 

 electromotive force in the aerial, due to the continually arriving trains 

 of waves, is able to act against the tapping and to keep the filings in 

 the tube in the conductive condition. Hence, the successful operation 

 of the arrangement requires attention to a number of adjustments, but 

 these are not more difficult, or even as difficult, as those involved in 

 the use of many telegraphic receivers employed in ordinary telegraphy 

 with wires. 



Mr. Marconi also introduced devices for j^reventing the sparks at 

 the contacts of the electromagnetic hammer from directly affecting 

 the tube, and also to prevent the electric oscillations which are set up 

 in the aerial from being partly shunted through other circuits than that 

 of the sensitive tube. We pass on to notice the remaining devices for 

 restoring the metallic filings tube to a condition of sensitiveness or 

 receptiveness. 



A method for doing this by alternating currents is due to Mr. S. 

 G. Brown.* The pole pieces of the coherer tube are made of iron, and 

 they are enveloped in magnetizing coils traversed by an alternating 

 electric current. Between these pole pieces is placed a small quantity 

 of nickel or iron filings, and under the action of the electromotive 

 force, due to an electric wave acting on them, may be made to cohere 

 in the usual fashion; but the moment that the wave ceases, the alter- 

 nating magnetism of the electrodes causes the filings to drop apart or 

 decohere. In place of the alternating current, Mr. Brown finds that 

 a revolving permanent magnet can be used to produce the alternating 

 magnetization of the pole pieces of the sensitive tube or coherer. 



The third method of causing the decoherence of the filings is that 

 due to T. Tommasina. He found that when a Branly tube is made 

 with filings of a magnetic metal, such as iron, nickel and cobalt, the 

 decoherence of the filings can be produced by means of an electro- 

 magnet placed in a suitable position under the tube.f The explana- 

 tion of this fact seems to be that, when an electric wave falls upon 

 the tube or when any other source of electromotive force acts upon it, 

 chains of metallic particles are formed, stretching from one electrode 



* British Patent Specilication, No. 19,710 of 1899. 



■f Comptcs Rendus, Vol. CXXVIII., p. 1225, 1889; Science Abstracts, Vol. 

 II., p. 521. 



