3 6o SCIENCE PROGRESS 



that a contact between aluminium and tellurium will act in 

 the same manner. 



A contact between copper and molybdenite, as G. W. Pierce 

 has shown, is likewise a rectifier, whilst Pickard has found 

 that a contact between zincite (a native oxide of zinc) and 

 chalcopyrite (copper pyrites) is an extremely good rectifier. 

 This rectification does not depend on thermoelectric action, 

 as the rectified current is generally in the direction opposite 

 to the current which would be produced by heating the junction. 

 R. H. Goddard has recently asserted that an oxide or sulphide 

 film of some kind is necessary for rectification and that the 

 contact between pure metals and pure non-metals in vacuo or 

 hydrogen is non-effective as a rectifier. 



It appears as if the film of oxide or sulphide or some 

 other impurities on the surface permits the passage of electrons 

 or ions through it more easily in one direction than the other 

 when the boundary surfaces are certain metals and non-metals. 

 In a large number of instances the direction of most easy 

 passage is such that electrons or negative ions seem to pass 

 more easily from the poor conductor (silicon, carbon, galena, 

 molybdenite, pyrites, etc.) to the good conductor (steel, copper 

 or gold) in contact with it but the tellurium-aluminium contact 

 is an exception to this rule. In any case, there is an asymmetry 

 of conduction at the boundary surface of many such contacts 

 between different classes of conductors which, when associated 

 in series with a telephone receiver, enables it to rectify trains 

 of electrical oscillations and to make audible groups of such 

 trains when coming at intervals of a few hundred a second. 



The discovery of these rectifying contacts in the course of 

 the search made for radio-telegraphic receivers has not only 

 opened up questions of great interest in connexion with the 

 conductance of electricity but has made it clear how great is our 

 ignorance as yet concerning so familiar an operation as the 

 movement of electricity through conductors. 



In addition to the crystalline or contact rectifiers referred 

 to, another type much used is the glow-lamp rectifier or oscilla- 

 tion valve, invented by the writer, which consists of a small 

 carbon filament glow-lamp having a metallic plate in its bulb 

 carried on a platinum wire sealed through the glass. When the 

 filament is incandescent, the space between it and the plate 

 has unilateral conductivity, negative electricity passing from 



