Page:Popular Science Monthly Volume 63.djvu/455

Rh 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 inductance of the different parts of the whole receiving apparatus. The 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 preventing 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 alternating 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 electromagnet placed in a suitable position under the tube. The explanation 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