676 Professor J. A. Fleming [June 4, 



carborundum — for example, hessite, which is a native crystalHne 

 telluride of silver or gold ; an anatase, which is an oxide of titanium ; 

 and molybdenite, which is a sulphide of molybdenum. 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 slight contact 

 between certain classes of conductors possesses a unilateral conduc- 

 tivity, 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 contact 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 oscillations are rectified or converted into more or less prolonged 

 gushes of electricity in one direction through the telephone. These 

 coming at a frequency of several hundred per second, corresponding 

 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 

 researches of Professor 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 radiotelegraphic detector, 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 metalHc 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 residual gas a unilateral conduc- 

 tivity, 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 men- 

 tioned, 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 



