(X.) Marine Communication.—The 1914 convention bound all the contracting nations to pass legislation to compel all ships that sail from one country to another to be fitted with a radiotele graphic installation if they have on board so or more persons in all, subject to a few exemptions. Moreover such ships carrying more than 25 passengers must maintain a continuous watch. Every ship so fitted must carry, besides its main installation, an emergency installation of approved design. Further, the master of any ship fitted with wireless who receives a call for assistance is bound to proceed to help those in distress if requested to do so. Since 192o, the ships of the British mercantile marine have been required to engage three operators for voyages other than coastwise taking more than 48 hours from port to port when carrying 200 or more passengers. Two operators must be carried by such vessels taking between 8 and 48 hours from port to port, and regulations have also been issued relative to the carrying of wireless watchers on board in place of one or more of the certified operators.
Until about the year 1922 nearly all ship and shore com. munication had been conducted by spark stations transmit ting on 450 and 600 metres wave-length. These signals inter fered greatly with the development of the new art of broad casting especially near great ports, and consequently much consideration was given to the introduction of more modern apparatus on ships. The larger passenger ships and shore stations gradually adopted continuous wave methods, employing either the arc or the valve, with a wave-length of about 2,000 metres. This step not only avoided interference with broadcasting but also enabled communication to be established across great dis tances and made multiple telegraphy possible when desired. Now adays one and the same shore station can utilise its antenna for transmitting signals on several wave-lengths simultaneously—say 1,800, 1,900, 2,000 and 2,200 metres—thus enabling that station to work with four ships at a time without interfering appreciably with any other service, such as popular broadcasting. The tendency of development in this direction is in favour of the installation of valve transmitters rather than arcs, and with such transmitters it is possible to operate very near to broadcasting wave-lengths without causing interference. These marine valve transmitters are now being manufactured by the principal firms in many countries, usually being equipped to transmit on about 600 metres and about 2,000 metres. Inasmuch as there are still many vessels sailing with apparatus incapable of receiving con tinuous wave signals, the transmitters just mentioned are pro vided with a simple form of rapidly vibrating interrupter which chops up the continuous waves so as to yield a musical note in the receiving apparatus of the older or smaller ships.
One result of the increasing use of continuous wave transmitters at sea is that telephony is now being tried as an accessory to telegraphy. In order to transmit speech from a continuous wave plant it is necessary to modulate the waves by aid of a microphone. This modification is described in that section of BROADCASTING dealing with transmitters. The range obtained by telephony is only
about a quarter of that obtained when the same plant is used for heterodyne telegraphy. In 1929, however, experiments were suc cessfully conducted in the field of ocean telephony which enabled Atlantic liners to maintain touch with both sides of the ocean throughout almost the entire passage. It ought to be noticed that the apparatus required and installed on ships for the purpose of receiving signals—either spark signals, telephony or interrupted continuous waves, or continuous wave telegraphic signals—is al most identical with that employed by the public in receiving broadcast matter. It may consist, for example, of a three valve amplifier having one high frequency amplifying valve, one detec tor valve and one low frequency amplifying valve, with suitable switches for cutting out one of these stages when signals are strong enough. Such receiving apparatus is described under BROADCASTING. The only difference between the receivers there described and those required at sea is introduced for the pur pose of receiving continuous wave signals by the heterodyne method. For this purpose two main alternatives exist. In the one, variously called the self-heterodyne, autodyne or endodyne apparatus, a coil in the anode circuit of the high frequency triode valve is made to act inductively upon a coil in the grid circuit of the same valve in such a way as to generate electrical oscilla tions within those circuits.
The frequency of these oscillations is determined by a closed tunable circuit in either the anode or the grid circuit of the valve, and is adjusted to be slightly different from the frequency of the incoming signal waves. In consequence the incoming waves "beat" with the locally generated oscillations. The frequency of the beat can be adjusted by altering the frequency of the locally generated oscillations, and can in fact be varied so as to consti tute, after rectification by the detector valve, an alternating electrical current in the final circuit of the apparatus, usually the head telephones of the listening operator. The other alter native method of heterodyne reception is named separate hetero dyne for the sake of distinction. In this method an entirely separate piece of apparatus consisting of a triode valve with a tunable circuit and with the anode circuit back-coupled to the grid circuit for ensuring the generation of electrical oscillations, is adjusted to generate oscillations of frequency slightly different from that of the incoming waves. This piece of apparatus, named the auxiliary oscillator, is brought near enough to the receiving apparatus to induce therein oscillations of its own fre quency, which interfere with the incoming waves and finally produce in the telephones of the operator an audible note of desired frequency. The loudness of this heterodyne note can be adjusted up to a certain limit by moving the auxiliary oscil lator nearer to or farther from the receiving apparatus. In both these alternative methods of heterodyne reception great mag nification is obtained by the introduction of the locally generated energy.