716 



TELEGRAPHY, WIRELESS. 



experiment on these waves was practically a dem- 

 onstration that they could be used for the transmis- 

 sion of signals ; for in every such experiment a 

 device was used for generating the waves and 

 another for detecting them at a distance, often with 

 solid objects intervening. The development of a 

 practical method, however, has depended largely on 

 the use of the " coherer " as a receiver. This device, 

 as developed by Branly in France (1890), Lodge in 

 England, and Onesti and Righi in Italy, depends on 

 the principle that a loose aggregation of metal par- 

 ticles becomes a conductor under the influence of 

 an electric wave, because, or at any rate as if, the 

 electric influence causes the particles to cohere con 1 

 tinuously. Mechanical disturbance causes them to 

 " decohere " and become again non-conducting. At 

 Oxford, in 1894, Dr. Lodge showed how coherers 

 could be used, and proposed to apply a mechanical 

 tapper or an electric trembler to effect decoherence. 

 The perfection of this device and of other devices 

 used in this kind of space telegraphy is due largely 

 to Marconi, an Italian, who was " the first to con- 



Fio. 3. THE TRANSMITTER. 



ceive and to patent the application of Hertzian 

 waves to telegraphy, apart from mere signaling," 

 and whose name is perhaps the most familiar one 

 before the public in connection with the whole sub- 

 ject. The transmitting apparatus now used by 

 Marconi is simply a device for generating Hertz 

 waves, consisting of a battery, A (Pig. 3), an inten- 

 sity coil, B, giving a 6-inch spark in air, and a pair 

 of brass knobs, C. When the current is made, sparks 

 pass between these knobs and give rise to the 

 desired electric wave disturbance, which travels 

 outward in all directions with the velocity of light. 

 For long-distance work four knobs are used, of 

 which the two inner are half immersed in vaseline 

 oil, giving greater energy to the spark and to the 



waves. The receiving apparatus (Fig. 4) 

 consists of a delicate coherer attached to a 

 glass rod, A B, a decoherer, N E, a battery, 

 G, a separate cell, F, wings, C and D, and a relay, 

 H, to which may be attached a bell, K, or a Morse 

 inker. Whenever the current is switched on at the 

 transmitting station, which may be miles away, the 

 electric wave that results strikes almost instantly 

 against the coherer in the receiving apparatus, caus- 

 ing it to conduct and thus closing the circuit and 



ringing the bell or actuating the Morse inker. At 

 the same time the electro-magnetic device E oper- 

 ates the tapper N, which strikes the glass tube, caus- 

 ing the filings to decohere, and thus preparing the 

 coherer for a second signal. The sensitive relay H 

 is used to throw into circuit the current from a bat- 

 tery strong enough to operate the decoherer and the 

 bell or inker; since the current that passes through 

 the coherer is usually not of sufficient strength to 

 do this work. The wave, then, affects the coherer, 

 which starts the relay, which in turn does the sig- 

 naling. The " wings " C and D are strips of copper 

 or brass about 1 by 12 inches, whose duty is to assist 

 in intercepting the electric wave and conducting it 

 to the filings in the coherer. The wave sets up 

 electric oscillations in these wings, which must ac- 

 cordingly be so adjusted in length as to respond 

 most effectively to the transmitter. The filings in 

 the coherer used in this case are 96 per cent, hard 

 nickel and 4 per cent, silver, with a trace of mer- 

 cury. The sensitiveness is increased by increasing 

 the proportion of silver. The filings are rather 

 coarse and must be of uniform size. 

 They are contained in a glass tube 2 

 inches long, into the ends of which en- 

 ter wires so connected that if they were 

 in electrical contact the circuit contain- 

 ing battery, relay, and bell would be 

 completed. The ends terminate in sil- 

 ver blocks fa of an inch apart, which 

 fit closely in the tube ; and the circuit 

 is therefore complete only when the 

 filings in the gap are in a condition to 

 allow the current to pass. The gap is 

 not packed with filings, but filled loose- 

 ly, so that the tapper N readily knocks 

 them apart. 



To prevent the high-frequency oscil- 

 lations set up in the receiver by the electric wuvcs 

 from running around the local battery wires, there- 

 by interfering with the working of the instrument. 

 Marconi uses " choking coils " (T T). which are small 

 coils having great self-induction, formed by wind- 

 ing about a yard of thin, well-insulated wire around 

 a core 2 or 3 inches long. 



In another form of receiver, devised by Dr. Oliver 

 Lodge, and now made in essentially the same form 

 under the name of " Miller and Woods's receiver." 

 the coherer is a small ebonite cup containing nickel 

 filings into which two supporting wires pass. The 

 decohering apparatus consists of two small n>d> 

 with a screw thread on them which, when actuated 

 by the signal bell, rub across the wires of the coherer. 

 The apparatus is very compact, oc- 

 cupying less than one cubic foot. 



The Marconi apparatus has under- 

 gone some important modifications 

 at the hands of an American engi- 

 neer, W. J. Clark. Mr. Clark em- 

 ploys for emitting and absorbing the 

 electric waves metal plates, which 

 must be of proper size and perfectly 

 parallel in order to get the best re- 

 sults. 



Wehnelt has recently devised a 

 new form of contact-breaker that 

 enables oscillations to be obtained 

 at the rate of over 1,000 a second, and sends out 

 trains of waves that follow each other with practi- 

 cally no interval. It is electrolytic in principle, 

 and consists of a glass vessel partly filled with 

 dilute sulphuric acid and having a negative elec- 

 trode of lead and a positive of platinum wire pro- 

 jecting from an insulating glass tube. When put 

 in a circuit at a pressure of 100 volts, together with 

 an induction coil, there is formed in the liquid be- 

 tween the two electrodes a voltaic arc which is very 



