Page:The New International Encyclopædia 1st ed. v. 20.djvu/683

WIRELESS TELEGRAPHY. 585 WIRELESS TELEGRAPHY. natiiig current is set up in the secondary ; since the .si'eoMiJary coil lias many turns of wire in it compared to the primary coil, the lov-vo]laf,'e current is transluniicd into one of high potential (see Induction Coil) having a frequency equal to the rate at wliieli the primary circuit is made and broken by the interrupter. This low-fre- quency, liigh-poti'ntial current charges the aerial wire and the wire leading to the earth with posi- tive and negative electricity, establishing a dif- ference of potential of several thousand volts. When this potential difl'erence is maximum the resistance of the air film of the spark-gap is broken down, and a spark passes between the brass balls forming the gap, and the difference of potential is equalized. The process of restoring the electrical equi- librium requires a very small fraction of a sec- ond, since the high-frequency currents oscillate from the top of the aerial wire, 8, to the earthed plate, 9, sometimes at the rate of a million times per second. Now electric oscillations transform their en- ergy into electric waves, and these electric waves are propagated through space with the rapidity of light to the receiver. When the electric waves impinge upon the aerial wire, 10, of the receiving system they are retransformed into electric oscillations, identical in frequency with those in the sending circuit, but their po- tential is very much reduced ; these enfeebled oscillations surging in the receiving circuit affect the coherer, breaking down its resistance, per- mitting the local current from the battery, 13, to flow tlirough the wave detector and telephone receiver or relay, 14, and indicating the imping- ing wave in the form of a dot or a dash, depend- ing upon its length ; it ceases, however, the instant the oscillations stop. History. In 1705 Salva, a Spanish physicist, suggested that it might be possible to charge the earth at llajorca with positive electricity and that at Alicante with negative electricity, when the attraction of the opposite changes would establish communication between the two cities. The earliest actual experiment by which it was demonstrated that an electrical connection could be estalilished between a transmitting and a receiving instrument with only water as a con- necting medium was performed by Siimmerring of Munich on .June 5, 1811; this he did by the dispersion or leakage method. The next step to- ward wireless telegraphy was made when Stein- heil accidentally discovered in 1838 that by grounding the terminals of a single telegraph wire the earth would act as a conductor for the returning electrical energy. Prior to this time the Morse wire telegraph employed a complete metallic circuit, which was evidently a modifica- tion of the leakage method. To ilorse appears due the credit of making the first practical test of wireless telegraphy based on the principles of the dispersion of an electric current, he having connected Governor's Island with Castle Garden, New York, a distance of a mile. Tn 1886 Amos E. Dolbear of Tufts College. Mass., evolved his electro-static method of wire- less telegraphy, by Avhich he was enaliled to trans- mit and receive signals'a distance of half a mile. Thomas A. Edison in 1801 took out a United States patent on what he termed an induction method, but it was based on the principles of electro-statics. Wireless telegraphy by true in- duction gave so little promise tluit no serious at- tempts were made to apply it until 1801, when John Trowbridge advocated the method and de- duced the conclusions necessary to be attained for transatlantic signaling, in 180fi, however, a new metliod was Ijrought to light by William Marconi, and although the principles underlying it were known, its practical apjilir'ation had not as yet been tested. This was the electric w.ave nujthod, and its history began with Michael Faraday's theory of the electro-magnetic origin of liglit in 184.'). This theory was deduced mathe- matically by James Clerk Maxwell in 18fi4, but it was not until 1888 that it received a physical demonstration, when Heinrich Hertz experi- mentally proved that electric waves followed exactly the same laws as light waves. But Hertz did more ; he showed how to produce these electric waves by purely physical means as well as how to detect them. His apparatus, especially his wave detector, was crude, consisting merely of a circlet of wire severed at a given point so that an air-gap formed a miscroscopic portion of it. His trans- mitter was not unlike those in use at the pres- ent time in wireless telegraphy; it consisted of an induction coil, battery, key. and oscillator. This was the real beginning of .wireless teleg- raphy, and from tliis time its development may be said to date. What was now needed to transmit signals to greater distances was a more sensitive detector, and this was forthcoming in 1890. when Eduard Branley constructed his radio-conductor or co- herer, consisting of a little glass tube contain- ing .some metal filings. This was a marvelously sensitive detector to the electric radiations. In 180.5 Professor Popoff employed the coherer in meteorological observations, and connected one of the coherer terminals to an aerial wire and the opposite terminal to the earth ; a relay, elec- tric bell, and l)attery completed the apparatus. Marconi in 1806 produced the first wireless telegraph capable of propagating and indicating electric waves over long distances. This he did by connecting one side of the spark-gap to an aerial wire and the opposite side to the earth. By adapting the receiver of Popoff. connecting an aerial and earthed wire to the receiver, and improving the coherer he was enabled to send messages a distance of 300 feet on his arrival in England in 1806; this distance he gradually ex- tended until now it is possible to signal without cal)les across the Atlantic Ocean. TiiEORT. The theoretical considerations upon which the electric wave method of wireless teleg- raphy is based may be divided into two parts, namely (a) the conversion of electric oscillations into electric waves and vice versa, and (b) the propagation of electric waves through space. That portion of the transmitter which includes the aerial and earthed wires and the spark-gap, 7, 8, 9 (see Fig. 61, is termed the oscillator si/s- tern, and that portion of the receiver comprising the aerial and earthed wires and the wave de- tector. 10, 11, 12, is termed the resonator system. When the spark passes between the balls of the spark-gap. 7. high-frequency high-potential cur- rents ore set up in the oscillator system: these oscillatory currents are not constant, but their