Dfckmker 1 moo 1 



K NOWLEDGE 



283 



tance at which the signals can be distinguished is re- 

 duced to neaxlv haJf that attained when they ai'c 

 employed. 



lu order to screen the receiver from the violent 

 surgings which would be set up when using the trans- 

 mitters at the same station, he enclosed the whole of 

 the i-eceiviug apparatus, with the exception of the re- 

 corder, in a metallic bos. As some of the waves picked 

 up bv the recorder would, by travelling along the leads 

 into the receiver, injure the coherer, he chokes off all 

 such effects bv interposing suitable choking coils between 

 the recorder connections and the terminals of the re- 

 ceiver. These choking coils consist of a few turns of 

 instJated wire wound in layers, each layer being 

 separated from the adjacent ones by means of sheets 

 of tinfoil in metallic connection with the enclosing box. 

 This earthed tinfoil prevents the waves from passing 

 inductively from one turn of the choking coil to the 

 next. The earthed terminal of the receiver is connected 

 to the box and need never be touched. Signor Marconi 

 also found that, unless provision was made against it, 

 the relay, the tapper, and the recorder all produced 

 disturbing effects on the receiver, but he got rid 

 of these effects by introducing suitable non-inductive 

 resistances, q, ])-, and s, in parallel with them, or, 

 as telegraphists say, he shunted them with these resist- 

 ances. This prevented all spaa'king at the contacts, and 

 sudden perturbations, or jerks, due to the local battery 

 current, all of which would otherwise produce disturbing 

 effects on the coherer. 



When it is desired that the receiver should only pick 

 up waves coming in a certain definite direction, the 

 arrangement shown in Fig. 7 is employed. This differs 



"^ — o^mm 



Fig. 7. — ilarconi EeceiTer with Parabolic Ecflector. 



from that shown in Fig. 6 only in the vertical wire and 

 earth connection being done away with, and replaced by 

 the two copper strips, k, k, the sizes of which must be 

 carefully adjusted so that the receiver may be in syntony 

 with the transmitted waves ; and in a parabolic cylin- 

 drical reflector being placed so that the coherer tube 

 lies with its axis in its focal line. 



My readers will observe the considerable similarity 

 between Marconi's apparatus and that of Popoff; al- 

 though I believe that, when Signor Marconi designed 

 his apparatus, Popoff s results were unknown to him. 

 Both used the coherer to actuate a relay, and thereby 

 bring into action a telegraph recorder, and both used 

 a tapper to cause decoherence. Popoff also anticipated 

 Marconi in the use of a vertical wire and earth con- 



§) I" 

 A 



8> 



nection on his receiving instrument, but does not seem 

 to have recognised its necessity on the transmitter. 



The use of the tall vertical wire on both transmitter 

 and receiver forms one of the most notable of Maixoni's 

 improvements, and the one which has perhaps played the 

 largest pai't in his successful transmission of signals 

 over long distances. A horizontal wire is no use, even 

 if added at the top of a long vertical wii'e, so as to keep 

 it at a great distance from the earth. The effect of 

 the long wire is to increase the length of the waves 

 generated in the ether, and, therefore, as was pointed 

 out in my first article, to augment their power of pene- 

 trating obstacles, the wave length being about four times 

 the length of the wire. The reason that it acts so 

 much better in a vertical position than in any other 

 is that that position is the one which is least favourable 

 to the production of induced oscillating currents in the 

 earth, which, if set up, must dissipate uselessly the 

 amount of energy required to excite them. 



Signor Marconi finds that a conductor with consider- 

 able capacity, such as a sheet of wire net, attached 

 to the top of the vertical wire by means of an insulating 

 rod, is to some extent equivalent to increasing the length 

 of the wire. He found experimentally that, if the 

 wires at the two stations are equal in height, the distance 

 to which signals could be transmitted was approximately 

 proportional to the squai-e of that height, the actual 

 maximum distance being somewhat in excess of that 

 calculated from this assumption. Professor Ascoli has 

 confirmed this result mathematically. 



One of the Marconi masts, 150 feet high, which was 

 erected at the South Foreland last year, is shown in 

 Fig. 8. This mast is now of historic interest, as being 

 the one which was used for the first transmission of 

 messages by the new system of telegraphy between 

 England and France, the French station being at the 

 village of Wimereux, near Boulogne, and at a distance 

 of 32 miles from the South Foreland. In place of using 

 a high mast the vertical wire might, where the oppor- 

 tunity exists, be suspended from the top of a cliff or of 

 a lofty building, and Mr. Marconi has in this way suc- 

 cessfully transmitted messages between Bournemouth 

 and Alum Bay in the Isle of Wight, a distance of about 

 14 miles. No mast was employed at the latter station, 

 the vertical wire being allowed to hang over the edge of 

 the cliff, the instrument and earth connections being 

 at the top, while the lower end of the wire, which was 

 about 100 feet long, hung free in space, the wire being 

 kept at a distance of about 30 feet from the face of the 

 cUff. 



I am not aware that any attempts have been made to 

 employ the Marconi apparatus with reflectors for greater 

 distances than two miles. Hertz found that to obtain 

 good results with reflectors they must be large compared 

 with the wave length, and the distance of the mirror 

 from the oscillator must not be less than a quarter of 

 the wave length, as clearly follows from what I explained 

 in my first article, that the emission point of the waves 

 is a quarter of a wave length from the vibrating source. 



It will be seen, therefore, that it would hardly be 

 practicable to employ reflectors in conjunction with high 

 masts for transmitting beams of rays in a given direction. 

 For example, with the vertical wire 150 feet long, such 

 as that in use at the South Foreland, the wave length 

 would be about 600 feet. The dimensions of the mirror 

 would therefore have to be large compared with this, 

 and placed at a distance not less than 150 feet from 

 the oscillator. 



The use of reflectors is, however, of considerable value 



