;6o 



NA TURE 



[July ii, 1907 



train are determined. Methods have now been devised for 

 creating undamped or nearly undamped oscillations. One 

 method suggested is by the use of a high-frequency 

 alternator, but the difliculty of constructing such a machine 

 for a sufficiently high frequency is very considerable, and 

 as yet only machines of small power have been made. 



Prof. Elihu Thomson in 1892 patented a plan for pro- 

 ducing a continuous current electric arc between metal 

 terminals which was acted upon by an air jet or magnetic 

 field. He found that when the metal poles or spark balls 

 were connected by a condenser in series with an inductance 

 high-frequency oscillations were created in the latter 

 circuit. 



Mr. Duddell showed in 1900 that continuous undamped 

 oscillations could be obtained when using, in place of the 

 spark balls, an electric arc made between solid carbons. 

 In 1903 Mr. Poulsen went a step farther. He formed 

 the arc between a carbon terminal (negative) kept in rota- 

 tion and a cold metal terminal (positive), and enclosed 

 them in an atmosphere of hydrogen or coal gas and placed 

 a strong magnetic field across the arc. The arc terminals 

 being then shunted by a small condenser and an induct- 

 ance, we have undamped high-frequency oscillations suit- 

 able for wireless telegraphy created in this latter circuit. 

 Prof. Fleming gave an explanation of the reason for this 

 effect, and showed that it was due to the form of the 

 characteristic curve of the metal carbon arc. He com- 

 pared the action to that of an organ pipe in which the 

 energy is supplied by a steady jet of air corresponding to 

 the continuous current arc, and this is made to set up 

 aerial vibrations in the resonant organ pipe, equivalent to 

 the condenser circuit, the vibrations in the pipe controlling 

 and drawing their energy from the air jet. In applying 

 the Poulsen method to wireless telegraphy, an antenna 

 circuit is connected either inductively or directly with the 

 condenser circuit. 



Experiments were shown illustrating the production of 

 high-frequency undamped electric oscillations in a long 

 wire wound on an ebonite rod. Incidentally it was proved 

 by an ingenious experiment with a neon vacuum tube that 

 the oscillations are not really continuous, but cut up into 

 irregular groups. In reference to the application of the 

 Poulsen method to wireless telegraphy, Prof. Fleming said 

 that the apparatus was more complicated and less easy to 

 use than that for spark telegraphy. It was not true that 

 undamped waves could not be picked up or tapped, and he 

 proved that such advantages as the use of undamped 

 waves in telegraphy might present were not due only to 

 the transmitter, but to the conjoint use of a slightly 

 damped receiver circuit. 



Prof. Fleming did not agree with the opinion, con- 

 fidently expressed in some quarters, that telegraphy by 

 undamped waves would destroy spark telegraphy. Further, 

 he pointed out that the electric arc was not the only 

 method for producing undamped waves. He said that for 

 some months past Mr. Marconi had been working out a 

 purely mechanical method of producing continuous trains 

 of electric waves suitable for wireless telegraphy, and had 

 obtained considerable success. The method was exceedingly 

 simple, and by means of it any existing station for spark 

 telegraphy could be converted easily into a station send- 

 ing out undamped electric-wave trains instead of inter- 

 mittent damped trains. 



The lecturer then passed on to explain some recent forms 

 of electric-wave detector. When the waves sent out from 

 the sending antenna fall on a receiving antenna they create 

 in it secondary high-frequency oscillations, and these are 

 detected by the use of some device called a wave detector. 

 In addition to the coherer and the Marconi magnetic 

 detector, the latter exclusively used in long-distance work, 

 a receiver called an electrolytic detector is now much used. 

 It consists of a vessel containing nitric acid in which is 

 placed a platinum plate and an exceedingly fine platinum 

 wire as the pair of electrodes. This cell is connected in 

 .series with a telephone and a voltaic battery, and when 

 electric oscillations are passed through the cell they annul 

 more or less the polarisation of the small electrode and 

 create an increase of current, which in turn makes an 

 audible signal in the telephone. 



Signals are thus sent on the Morse code by interrupt- 

 ing the oscillations in the sending antenna. It has, how- 



NO. 1967, VOL. 76] 



ever, recently been found that this electrolytic detector, in 

 conjunction with undamped-wave trains, affords a means 

 of transmitting, not merely dots and dashes, but articulate 

 speech sounds, and hence we have now electric-wave wire- 

 less telephony as an accomplished fact. Between Nauen 

 and Berlin, a distance of about sixteen miles, successful 

 experiments were recently carried out. From the trans- 

 mitter continuous undamped waves of constant wave- 

 length are sent out. The antenna is shunted by a micro- 

 phone, so that words spoken to its diaphragm vary the 

 intensity of the wave train, but not its wave-length. At 

 the receiving station an electrolytic detector is used, and 

 the words spoken to the transmitter are reproduced at the 

 receiving telephone. It is almost practicable at the present 

 moment to speak audibly across the English Channel by 

 wireless telephony, and within the bounds of possibility 

 that at some time we may telephone without wires to a 

 ship in the middle of the Atlantic. 



Another new form of electric-wave detector is a glow- 

 lamp detector. A small carbon filament glow lamp has a 

 metal cylinder placed in its bulb so as to surround, but 

 not touch, the filament. This cylinder is connected to 

 third terminal by a wire sealed through the glass. When 

 the filament is rendered incandescent by a continuous 

 current it throws off negatively charged corpuscles or 

 electrons. The space between the filament and the 

 cylinder will pass negative electricity from the filament 

 to the cylinder, but not in the opposite direction. If, 

 then, the oscillatory current in the receiving antenna, or 

 secondary currents induced by them, are passed through 

 the bulb from the carbon filament to the cylinder, only 

 one constituent of the oscillation passes, or the bulb 

 rectifies the oscillations. We can then insert in series with 

 the bulb either a telephone or galvanometer sensitive only 

 to continuous currents and cause it to be affected. 



This glow-lamp detector or oscillation valve has already 

 proved itself to be a most sensitive long-distance receiver 

 for wireless telegraphy. It is of great use in connection 

 with undamped electric waves, as it affords a ready means 

 for converting the high-frequency alternating current 

 created in the receiving antenna into a continuous current 

 detectable by a galvanometer or telephone. Prof. Fleming 

 showed by an experiment that it enabled him to revive in 

 another form electromagnetic induction telegraphy. 



In the form in which this older system of wireless 

 telegraphy was practised by Trowbridge, Preece, Lodge, 

 and -others, a closed primary circuit was traversed by a 

 rather low-frequency alternating current, viz. one of which 

 the frequency lay within the limits of audible sound. In 

 the modification proposed, a closed primary circuit earthed 

 at one point is traversed by a very high-frequency current 

 produced by the electric arc or other mechanical method. 

 With high frequencies, say of 100,000 or so, no disturbance 

 would be created in other earthed neighbouring telephonic 

 or telegraphic circuits. The receiving circuit is a similar 

 closed and syntonised circuit having a glow-lamp detector 

 or oscillation valve and telephone inserted in it. The 

 primary circuit affects the secondary circuit by magnetic 

 induction, and also acts like a Fitzgerald closed magnetic 

 nscillator, and throws off magnetic waves. With these 

 modifications it is possible that induction telegraphy may 

 be extended in range and free from the objection of dis- 

 '.urbing neighbouring telephonic circuits. 



The lecturer then passed on to consider some advances 

 made in directing electric waves in any required direction. 

 .\\l methods employing mirrors or reflectors are out of the 

 question when long telegraphic electric waves are under con- 

 sideration. Two methods at the present time presented prac- 

 tical advantages. In one, due to Mr. Marconi, the vertical 

 upright antenna w-ires arc replaced by antennas having a 

 short part vertical and their greater part horizontal. Such 

 a bent antenna radiates best in the opposite direction to 

 that in which its free or insulated end points. Hence, in 

 accordance with the law of exchanges, which applies to 

 rlectric radiators, a bent antenna absorbs best electric 

 waves coming from a direction opposite to that to which 

 its free end points. Making use of such bent receiving 

 and transmitting antcnnse, Mr. Marconi has been able to 

 limit the radiation in undcsired directions, and also to 

 locate the direction of invisible sending stations. 



.Another method, different in principle, has been devised 



I 



