DKCEMBtK 9, 1909] 



NA TURE 



171 



has already been made by H. Brandes, viz. that any 

 materials such as the crystals mentioned, or ionised gases, 

 which do not obey Ohm's law as regards the independence 

 of conductivity on impressed voltage can be used as radio- 

 telegraphic receivers. It is necessary to be able to test 

 the relative sensibility of detectors to know whether any 

 new form is an improvement. It is not always possible 

 for an inventor to get these tests made at real wireless 

 telegraph stations. Moreover, it is no use 'to test over 

 short distances, because then all detectors appear to be 

 equally good. I have found, however, that we can make 

 these comparative tests very easily within quite moderate 

 distances by employing closed sending and _ receiving 

 circuits which are poor radiators. All the devices called 

 wave detectors are really only oscillation detectors, and 

 we can therefore test their value simply by ascertaining 

 how feeble an alternating current or alternating voltage 

 they will detect. If we, then, set up in one place a 

 square circuit of wire a few feet inside, and complete the 

 circuit by a condenser and a spark-gap, _ we can set up 

 oscillations in it by means of an induction coil. I find 

 that it is necessary to enclose the spark-gap in a cast- 

 iron box, and to blow upon the spark with a jet of au- 

 to secure silence, absence of emission of electromagnetic 

 waves direct from -tjie spark balls, and constancy in the 

 oscillatory circuit. I then set up, a few score or few 

 hundred feet away, a similar tuned closed oscillatory 

 circuit, and I connect the oscillation detector to be tested 

 either in this circuit or as a shunt across the condenser. 

 The closed receiving circuit is so constructed that it may 

 be rotated round either of three axes. It is then gener- 

 ally possible to find some position of the receiving circuit 

 such that no sounds are heard in a telephone -connected 

 to a highly sensitive detector associated with the circuit. 

 This position is called the zero position. If the receiving 

 circuit is rotated round some axis, it begins at a certain 

 displacement to receive signals, and the angle through 

 which it has to be turned is a measure of the insensibility 

 of the particular oscillation detector being used. I find, 

 for instance, that it is quite easy to take one of my oscilla- 

 tion valves, a magnetic detector, an electrolytic detector, 

 a crystal detector, or any other type, and arrange these 

 in order of their sensibility by means of the device 

 described. 



Sensibility is not, however, the only virtue which a wave 

 detector should possess. It is important that it should be 

 simple, easily adjusted, and not injured by the chance 

 passage through it of any unusually large oscillatory 

 currents. Another quality which is desirable is that it 

 should be quantitative in its action, and that any change 

 in the amplitude of the wave received should be accom- 

 panied by an equal change in the current which the 

 detector allows to pass through the telephone. A quanti- 

 tative oscillation detector, then, enables not merely signals, 

 but audible speech to be transmitted. In other words, it 

 can effect wireless telephony. The difficulties, however, 

 in connection with the achievement of wireless telephony 

 are not so much in the receiver as in the transmitter. 

 We have to obtain, first, the uniform production of per- 

 sistent electromagnetic waves radiated from an antenna, 

 and next we have to vary the amplitude of these electric 

 waves proportionately to, and by means of, the aerial 

 vibrations created by the voice speaking to some form of 

 microphone. We cannot employ an intermittent spark 

 generator, because each spark would give rise to a sound 

 in the telephone, and these sounds, if occurring at regular 

 intervals, would produce a musical note in the telephone. 

 If, however, we make the sparks run together into what 

 is practically a high-voltage arc taking a small current, 

 then, in an oscillatory circuit shunted across this arc, we 

 have set up persistent high-frequency oscillations, as first 

 achieved by Mr. Duddell. 



We can greatly increase the energy of the oscillations 

 by immersing the arc in a strong transverse magnetic field 

 and also in a hydrocarbon gas, as shown by Poulsen, or we 

 may employ a number of arcs in series. E. Ruhmer has 

 lately also employed a high-tension arc between aluminium 

 electrodes (Fig. 22), shunted by a condenser and induct- 

 ance as a means of generating persistent oscillations. As 

 an alternative, it is possible to create them by a mechanical 

 method, viz. by a high-frequency alternator, subject, how- 



NO. 2093, VOL. 82] 



ever, to certain limitations as to frequency. Both these- 

 types of generator have their advantages and practical, 

 objections. There is good evidence that radio-telephony 

 has been accomplished over distances of too miles or more 

 by each of these methods in the hands of experts, but 

 what is now required is the reduction of the apparatus to. 

 such simple manageable and practical form that it can be 

 applied in regular work. The wave-generating apparatus, 

 must be capable of producing uniform persistent oscilla- 

 tions of high voltage and frequency, not less than 30,000 

 or 40,000 per second, or at least above the limits of audi- 

 tion, and the amplitude of these oscillations must be 

 capable of being varied by some form of speaking micro- 

 phone placed in the oscillation circuit or in the radiating; 

 antenna, or in a secondary circuit coupled to it. No 

 ordinary simple carbon microphone will safely pass 

 sufficient current for this purpose. A type of multiple 

 microphone has been used successfully, and also a duplex 

 microphone, the invention of Ernst Ruhmer. 



It is not, however, possible to speak of radio-telephony 

 at the present time as having reached the same level of 

 practical perfection as radio-telegraphy ; but the possi- 

 bilities of it are of such a nature that it will continue to. 



persistent Oscillations for Ratliotelephony. 



producin:; 



attract the serious attention of inventors. This is nob 

 the place to enter into a full discussion of the causes which 

 limit submarine telephony through cables, but there are 

 well-known reasons in the nature of submarine cables as 

 at present made which impose very definite limits upon it, 

 owing to what is called distortion of the wave form. 

 Electric wave telephony is free at least from this dis- 

 advantage, and if (as has been asserted) arc generators 

 can be made self-regulating and capable of being worked 

 for hours automatically, or even for ten minutes without 

 being touched, then the remaining difficulties with the 

 microphone are not insuperable. 



Time does not permit of the discussion of the many 

 other points in connection with radio-telegraphy and tele- 

 phony which have been the subject of recent work. Much 

 attention has been paid lately to methods of cutting out 

 atmospheric signals due to natural electrical discharges 

 in the atmosphere, which are troublesome disturbers of 

 the astherial calm necessary for radio-telegraphy. Con- 

 siderable thought and expenditure have been necessary to 

 discover means for overcoming the difficulties of long- 

 distance transmission by daylight, and also those arising 

 from the cross-talk of other stations. Much also has been 



