This system of non-interfering multiplex wireless telephony is not, however, limited to the sending of three messages or conversauons simultaneously in the same neighborhoods. For, by using carrier waves of say, 189 and 239 meters in length, in addition to the wave length of 150 meters, and by modulating each of these two additional carrier waves with intermediate frequences of 25.000, 35,000 and 45,000 cycles it will be feasible to carry on nine separate wireless Conversations simul tubes VT', the output circuits of which in turn are coupled through transformer RC' to the input circuits of a battery of three power tubes VT'. Similarly the output circuits of the latter tubes are coupled through the trans.. former RC' to the aerial A, from which the modulated high frequency oscillations ampli fied and increased in power are radiated into taneously between the ships equipped and placed as indicated in the foregoing.
Tests with apparatus somewhat similar to that just described showed that two-way wire less telephone conversations could be carried on between ships at a distance of about 40 miles, the power of the outfits being low. The limit of speaking distance was about 150 miles. By two-way conversation is meant the ability to converse in either direction, one speaker at a time on one circuit. Apparatus to permit this two-way conversation was provided on the ships. The object in resorting to the short wave lengths of 150 and 189 meters in the tests just described was to avoid interference or clashing with the regular wireless telegraph operation of the ships of the navy which use wave lengths of 600 to 1,200 meters.
In Fig. 13 is shown an arrangement of space. BB in the figure indicate the usual heat ing and potential batteries. The power tubes, it will be noted, are connected in quantity or multiple and not in series or cascade as in the case of amplifier tubes. See Fig. 6.
Long Distance Wireless Telephony.—An epochal series of experiments in long-distance wireless telephony was in 1915 conducted by the American Telephone Company in conjunc tion with the Western Electric Company and representatives of the United States navy and the French army. The most notable of these tests were perhaps those made between ton, Va., and the Eiffel Tower, Paris. Tests had been previously made between Arlington and Darien, 2,100 miles oversea, and between Arlington and Mare Island, near San Fran cisco, 2,500 miles overland. In the Arlington Darien 'experiments speech was transmitted amplifying and power tubes such as referred to in the description of Fig. 11. G is a gen
erator of high frequency oscillations which after transformation by coils RC may be modulated by the microphone transmitter T. These modulated oscillations are amplified by the tube VT, the output circuit of which is coupled through transformer RC' to the input circuits of a battery of two power vacuum from New York City by land lines to Arling ton, whence it was transmitted by wireless telephony to Darien. In the Arlington-Paris experiments difficulty was found in obtaining the use of the Eiffel Tower station, owing to the exigencies of the Great War which was then in progress. In these tests the Eiffel Tower station was used only for receiving. On 22 Oct. 1915, however. speech transmitted from Arlington was clearly received in Paris, a dis tance of 3,600 miles. The radio-wave length employed was 6,000 meters. The antenna cur rent employed at the Arlington station was about 50 amperes. The speech transmitted on this and other occasions was also received, and the voice of the speaker clearly recognized by the wireless operator in Honolulu, 4,500 miles from Arlington.
The general plan of transmission adopted at Arlington was somewhat similar to that indicated in Fig. 12, but of course the number and capacity of the amplifier and power tubes used at Arlington was greatly in excess of that used in shipboard or ordinary wireless tele phone practice.
The Future of Wireless Telephony.— In view of the rapid development of wireless telephony within the past 8 or 10 years it would be unwise to attempt to place a limita tion upon the extent of the use of this art within the next, decade. The possibility of transmitting wireless telegraph messages a dis tance of 12,000 miles, namely, from Carnarvon, Wales, to Sydney, Australia, has been demon strated (2 Oct. 1918). Likewise the possibility of transmitting speech by ether waves a dis tance of 4,500 miles has been shown. These results are obtainable by the use of the vacuum tube oscillators and amplifiers. As a distance of 12,000 miles is half way around the earth and as the ether waves in traveling that distance traverse both sides of the earth, it follows that this is equivalent to transmitting a message from one point to any other point on the earth. Since also it appears evident that by means of the three-electrode vacuum tube speech may be transmitted to any distance reached by radio-telegraph waves, it would seem that un der favorable conditions it may be said that world-wide wireless telephony is now a possi bility.