476 Prof. J. A. Fleming. Conversion of Electric [Jan. 24, 



" On the Conversion of Electric Oscillations into Continuous 

 Currents by means of a Vacuum Valve." By J. A. FLEMING, 

 M.A., D.Sc., F.R.S., Professor of Electrical Engineering in 

 University College, London. Received January 24, Read 

 February 9, 1905. 



An electric oscillation being an alternating current of very high 

 frequency, cannot directly affect an ordinary movable coil or movable 

 needle galvanometer. 



Appliances generally used for detecting electric waves or electric 

 oscillations are, therefore, -in fact, alternating current instruments, and 

 must depend for their action upon some property which is inde- 

 pendent of the direction of the current, such as the heating effect 

 or magnetizing force. The coherer used in Hertzian wave research 

 is not metrical, since the action is merely catastrophic or accidental, 

 and bears no very definite relation to the energy of the oscilla- 

 tion which starts it. Even the demagnetising action of electric 

 oscillations, though more definite in operation than the contact action 

 at loose joints, is far from being all that is required for quantitative 

 research. It is obvious it would be an advantage if we could utilise 

 the direct current mirror galvanometer for the detection and measure- 

 ment of feeble electric oscillations. This can be done if we can 

 discover a medium with perfect unilateral conductivity. 



Some time ago, I considered the use of the aluminium-carbon 

 electrolytic cell with this object. It is well known that a cell con- 

 taining a plate of aluminium and carbon, immersed in some electrolyte 

 which yields oxygen, such as dilute sulphuric acid or an aqueous 

 solution of any caustic alkali, or salt yielding oxygen, has a unilateral 

 conductivity within limits. An electric current under a certain 

 electromotive force can pass through the cell from the carbon to the 

 aluminium, but not in the reverse direction. 



This action has been much studied and is the basis of many technical 

 devices, such as the Nodon electric valve. 



The electrochemical action by which this unilateral conductivity is 

 produced involves, however, a time element, and after much experi- 

 menting I found that it did not operate with high frequency currents. 

 My thoughts then turned to an old observation made by me in 1889, 

 communicated to the Royal Society, amongst other facts, in a Paper 

 in 1889, and also exhibited experimentally at the Royal Institution in 

 1890.* This was the discovery : that if a carbon filament electric 



* See 'Koy. Soc. Proc.,' vol. 47, p. 122, 1890, "On Electric Discharge between 

 Electrodes at different Temperatures in Air and High Vacua," by J. A. Fleming, 

 communicated December 16, 1889 ; see also ' Proceedings of the Eoyal Institution,' 



