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to the wave length of the wave to be received. This frame is then placed with its plane vertical and in the direction of the sending station. As the incident electric waves sweep over it they set up in the wire very feeble electric oscillations. If a cascade thermionic amplifier is then connected to the terminals of the receiving con- denser and appropriate tuning carried out the signals will be heard in the receiving telephone. It is possible to make one small storage battery of three cells provide the electric current for incandescing the filaments, and one battery of 40-50 cells provide the plate cur- rents for all the valves. Very compact and portable multiple valve receivers of this type have been constructed for use in aircraft and for reception of time signals from distant radio stations.

FIG. 8. Marconi Co. 's type 55; thermionic amplifier with six amplifying valves and one detector as used in wireless telegraphy and telephony. (By permission of Marconi's Wireless Telegraph Co., Ltd.)

Directional Wireless. The frame aerial has the important quality of being directive; that is, it tells us the direction in which the incident waves are travelling. Hence if two receiving stations at a known distance apart are provided with directive aerials, and if they simultaneously observe the direction of the ar- riving waves from one transmitting station, which may be on an aircraft or ship, these observations laid down on a chart will enable them to fix the position of the source of the signals. In this manner the position of aeroplanes lost in the clouds or ships in the fog may be found and their exact position communicated to them. There was a considerable development of this directive radiotelephony during the World War of IQI4-8. 1 It has been found that there are peculiar difficulties in practising this direc- tion finding at or about the times of sunrise and sunset.

In place of employing a movable frame aerial two fixed nearly closed circuit triangular aerials can be erected with their planes at right angles, and a resonant receiving circuit can be arranged to have a coil which is capable of rotation round a vertical axis but so as to be coupled inductively to both the fixed aerials by coils in the two aerial circuits. If electric waves fall on the aerials and if the movable coil of the receiving circuit is rotated into the azimuth in which it receives signals most loudly the direction of the plane of that coil will determine the line of direction of the transmitting station. It is possible by special arrangements to determine the direction along this line in which the electric waves travelling. Many coast radio stations are now provided with direction finding aerials, and ships can call up these stations by wireless when in proximity, in case of fog, and have their bearings and exact position given to them. In another method of direction finding the coast station sends out a revolving beam of radiation which has a sharply marked point of zero radiation. The time of revolution of this beam is known, and also the instants when the zero radiation is in the true north and south direction at the sending station. Hence by observing the instants at which the zero radiation is observed at the ship, the ship's bearing with regard to the station can be determined. The station sends out time signals by which to correct the ship's chronometer.

1 See " Direction and Position Finding," by H. J. Round, Journal Inst. Elec. Eng. London 1920, vol. Iviii, p. 224; also J. J. Bennett, Nature, May 19 1921, vol. cvii, p. 363.

Another ingenious application of radiotelegraphy has been made by Prof. J. Joly to enable ships to find their position in fogs and avoid collisions. For details the reader must be referred to his paper in the Proceedings of the Royal Society of London, vol. xcii., A, 1915-6, pp. 170, 176, 252, and also to a paper by H. C. Plumer on P- 377 in the same volume. See also J. Joly, Proc. Roy. Soc. Lond., vol. xciv. A, 1918, p. 547.

Wireless Telephony. The above described improvements in the production and detection of continuous electric waves had by 1921, within a few years, placed wireless telephony on a thoroughly practical basis. It is unnecessary to describe various experimental feats which had been achieved at intervals in this art of radio- telephony, in which the Pouhen arc or some modification was employed to generate the continuous waves (C.W.). All practical radiotelephony now involves the use of the thermionic valve both as a generator of C.W. and as detector.

The general principles of this method are as follows: one or more three-electrode valves are employed in which the plate and grid circuits are inductively coupled so as to generate continuous oscillations. The plate circuit is also coupled inductively to a radiating aerial wire and continous waves sent off. The ampli- tude of these waves has then to be modulated in accordance with the wave form of a speech sound. This is done by means of another three-electrode valve called the control valve. The latter has the secondary circuit of an induction coil connected between its grid and filament, and in the primary circuit is a microphone transmitter and a voltaic cell or two. Hence if speech is made to the transmitter the potential of the grid is varied in accordance with the wave form of the speech. The plate current of this control valve is caused to act upon the plate current or grid potential of the oscillating valve so as to modulate the resulting high frequency oscillations also in accordance with the wave form of the speech made. At the receiving end the received oscillations are amplified by a series of valves and then rectified and passed through a Bell magneto-telephone. The speech sounds are then reproduced by the receiving telephone.

The advantage of this method is that only the ordinary standard telephone transmitter and receiver as used in telephony with wires are employed. To obtain the necessary high plate potentials in the oscillating valves we can either use voltaic batteries (dry cells) or else a small high tension direct current dynamo (1,000-2,000 volts), or else we can rectify a low frequen- cy high tension alternating current by one or more Fleming valves. For aeroplane wireless telephony the plate voltage is supplied by a small dynamo driven by a wind screw which is set in action when the aeroplane flies. A large number of schemes for valve circuits for wireless telephone have been devised. During the war a great amount of ingenuity was expended in devising compact light weight sets of radiotelephone transmitters and receivers for use in aircraft and in the field (see figs. 9 and ga). A problem of practical importance is that of two-way radiotelephony enabling two communicators to speak and hear simultaneously or to " cut in " or interrupt each other as can be done in ordinary telephony. If a single aerial wire has to be switched over from transmitter to receiver there is always risk of confusion owing to both operators trying to speak or listen at the same moment.

In the case of ground stations a practical solution is to use two wave lengths differing say by 5%. At each station there is a transmitter and a receiver say 100 yd. apart. One transmitter is tuned to the distant receiver but the wave length of the home receiver, which is tuned to the distant transmitter, differs by 5%. Each operator then speaks and listens on a different wave length and can " cut in " as he likes.

This method is, however, not applicable in the case of aeroplanes or ships for want of space. One suggested solution is that called the " quiescent aerial." The plate voltage of the oscillating valve is not supplied by a high voltage battery but at most by a few cells, and the remainder of the plate voltage is created by the rectification by the valve of the speech cuyents induced in the secondary circuit of the microphone transformer. In this case continuous waves are not thrown off from the aerial except in the act of speech to the microphone, and the receiver can then