WIRELESS TELEGRAPHY, 



265 



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Fk;. 2. — A Braiilv colu'ivr. 



tube and niaking- them part of an ordinary electric circuit (see tig. 2). 

 AVhen electric waves are set up in the iieighl)orhood of this circuit, 

 electromotive forces are generated in it, which !i})])eai' to l)ring tiie 

 tilings more closely togethtu' — that is, to cohere- and thus tlieir elec- 

 trical resistance decreases, from which cause this piece of ai)paratus — 

 the tu))e and its tilings-^is termetl a ''"coherer/' Hence, the receiv- 

 ing instrument, (I, in tlu^ tigure, 

 wliieh may be a galvanometer or 

 a telegraph relay, that normally 

 would not manifest any sign of 

 current from the small battery, 

 B, will be o])erated when electric 

 oscillations are set up. Profes- 

 sor Branh' further found that 

 when the tilings had once cohered 

 they retained their low electrical resistance until shaken apart, for 

 instance, by tapping on the tube. 



In 1894 Dr. O. J. Lodge showed that the Branly coherer could be 

 employed to transmit telegraphic signals, and in order that the tilings 

 mig-ht not remain '"cohered'' after tlie cessation of the electric oscilla- 

 tions, he devised a mechanical ''tapper." on the principle of the 

 common electric doorl)ell, the hanmier of which was caused to tap 

 the glass tube as long as the electric oscillations continued (tig. '.'>). 

 The tilings thus virtually take the place of a key in the ordi- 

 nary telegraph circuit. In the normal state the key is open; in 

 the presence of electric oscillations the key is closed. Thus, 

 by opening and closing the key for a longer or shorter period, 

 PQ^gpgp signals corresponding to dots and 



dashes may be i)roduced. In othiM' 

 words, by setting u|) electric oscil- 

 lations for periods of time corres- 

 pondijig to dots and dashes, mes- 

 sages may be transmitted, and if 

 at the receiving station a record- 

 ing instrument (controlled by the 

 ^„„ _ coherer), such as is used, ft)r in- 



BATTERV EARTH . , 



Fl.;. 3.-I.()(lgo Hipper and PoppoiY vertical wire stauce, ill tllC ^Vheatstone auto- 



"'^'^^'^'''- matic telegrai)h system," be pro- 



vided, a record of the message in dots and dashes is obtaintnl. .Viul 

 this, in short, is what is done in wireless telegraphy. 



In 1895-96 Poppoti' and others utilized the coherer to show the 

 existence of atmospheric electricity, using for the purpose a vertical 

 wire connected to the coherer, as shown in tig. 3. 



« Described in the present author's American Telegraphy. 



