NA TURE 



[December 9, 1909 



a sulphide of moh'bdcnum. As regards the origin of this 

 curious unilateral conductivity, it seems clear that it is 

 not thermoelectric, but at present no entirely satisfactory 

 theory of the action has been suggested. 



A number of forms of oscillation detector have recently 

 been invented which depend on the curious fact that a 



Applied VoUagf. 

 Stic Curve of Rarefied Ga 

 Electrode. 



Ionised by Hot Negati\ 



slight contact between certain classes of conductors 

 possesses a unilateral conductivity, and can therefore 

 rectify oscillations. One such detector, now much used in 

 Germany, consists of a plumbago or graphite point pressed 

 lightly against a surface of galena. It has been found by 

 Otto von Bronk that a galena-tellurium contact is even 

 more effective. To the same class belongs the 

 silicon-steel detector of Pickard. If such a con- 

 tact is inserted across the terminals of a condenser 

 placed in the receiving circuit, and if it is also 

 in series with a telephone, the trains of oscilla- 

 tions are rectified or converted into more or less 

 prolonged gushes of electricity in one direction 

 through the telephone. These, coming at a fre- 

 quency of several hundred per second, correspond- 

 ing to the spark frequency, create a sound in the 

 telephone, which can be cut up by the sending 

 key into Morse signals. According to the re- 

 searches of Prof. Pierce and Mr. Austin, it seems 

 clear in many cases that this rectifying action is 

 not thermoelectric, since the rectified current is in 

 the opposite direction to the current obtained by 

 heating the junction. 



I may, then, bring to your notice some recent 

 work on another form of radio-telegraphic de- 

 tector, which I first described to the Royal Society 

 about five years ago under the name of oscillation 

 valve. It consists of an electric glow-lamp, in the 

 bulb of which is placed a cylinder of metal, which 

 surrounds the filament but does not touch it. This 

 cylinder is connected to a wire sealed through the 

 glass. Instead of a cylinder, one or more metal plates 

 are sometimes used. The filament may be carbon or a 

 metallic filament, and I found some year or more ago 

 that tungsten in various forms has special advantages. 

 The bulb is exhausted to a high vacuum, but, of course, 

 this means it includes highly rarefied gas of some kind. 

 When the filament is rendered incandescent it emits 

 electrons, and these electrons or negative ions give to the 

 re.sidual gas a unilateral conductivity, as shown by me in 

 a Friday evening lecture given here nineteen years ago. 

 Moreover, the ionised gas not only possesses unilateral 

 conductivity, but its conductivity, like that of the crystals 

 just mentioned, is a function of the voltage applied to it. 

 Hence, if we apply an electromotive force between the hot 

 filament and the cool metal plate, we find that negative 

 electricity can pass from the filament to the plate through 

 the ionised gas, and that the relation between the current 

 and voltage is not linear, but is represented by a charac- 

 teristic curve bending upwards, which has changes of 

 curvature in it (Fig. i8). The sharp bend upwards at one 

 place implies a large increase in the current corresponding 

 to a certain voltage, which means that, corresponding to 

 a certain potential gradient, and therefore velocity of the 

 electrons, considerable- ionisation of the residual gas is 

 beginning to take place. The current, however, would not 

 increase indefinitely with the voltage, but would before 

 long become constant or saturated. 

 NO. 2093, VOL. 82] 



It will be seen, therefore, that at points on the curve 

 where there is a bend or change of curvature, the second 

 differential coefficient of the curve may have a large value. 

 Hence, if we consider the current and voltage correspond- 

 ing to this point, it will be seen that any small increase in 

 the voltage increases the current more than an equal small 

 decrease in voltage diminishes it. If, then, we super- 

 impose on a steady voltage corresponding to a point of 

 inflexion of the curve an alternating voltage, the average 

 value of the current will be increased. This, then, points 

 out two ways in which this oscillation valve or glow-lamp 

 can be used as a radio-telegraphic detector. First, we may 

 make use of the unilateral conductivity of the ionised gas 

 in the bulb and employ the glow-lamp with cylinder 

 around the incandescent filament, as a rectifier of trains 

 of oscillations to make them effect a galvanometer or tele- 

 phone. This method was described by me in papers and 

 specifications in 1904 and 1905. In that case the valve is 

 arranged in connection with a receiving antenna, as shown 

 in Fig. 19, and used with a galvanometer or telephone. 

 Mr. Marconi subsequently added an induction coil and 

 condenser, and employed in 1907 the arrangements shown 

 in Fig. 20. In this case the trains of oscillations set up 

 in the antenna could not by themselves affect a galvano- 

 meter or a telephone, but, when rectified by the valve, 

 they become equivalent to an intermittent unidirectional 

 current, and can then affect the telephone or a galvano- 

 meter, or any instrument for detecting a direct current. 



On the other hand, we may take advantage, as I have 



Connections for O: 



Detector. 



more recently shown, of the non-linear form of the 

 characteristic curve. In other words, of the fact that the 

 conductivity of the ionised gas is a function of the voltage 

 applied to it, and in the second method the valve and 

 receiving circuits are arranged as shown in Fig. 21. In 

 this case we have to apply to the ionised gas a unidirec- 

 tional electromotive 

 force which corresponds 

 to a point of inflexion 

 on the characteristic 

 curve, and then to add 

 to this voltage the 

 alternating voltage of 

 the oscillations set up 

 by the incident electric 

 waves in the receiving 

 circuit. The result is 

 to cause a change in 

 the average value of 

 the current through 

 the telephone, and 

 therefore to produce a 

 sound in it, long or 

 short, according to the 

 number of trains of 

 waves falling on the 

 antenna. This last method, then, requires the application 

 in the telephone circuit of an accurately adjusted steady 

 electromotive force, not any electromotive force, but just 

 that value which corresponds to a point on the character- 

 istic curve at which there is a sudden change of curvature. 

 At this point we may notice a broad generalisation which 



G. 21. — Connections for O.S{ 

 used as a Radiotelegraph I< 



