April, 1938 THE WIRELE SS ENGINEER 213

(5) In August, critical distance power 4. Salzberg and Haeff, R:C.A. Review, Vol. II, No. 3, January' 1935, 1938, pp. 336-374. valves were first marketed by The High Vacuum Schade, Proc. I.R.E., Vol. 26, No. 2, February, 1938, Valve Company, of III, Farringdon Road, E.C.r pp. 137-181. (England). The valves were shown, and one was 5. Gill, Phil. Mag., May, 1925, pp. 993-1005. operated in comparison with a , on the Tonks, Phys. Rev., October, 1927, pp. 501-511. " Hivac " stand at Radiolympia that year. Background Noise (6) The now acknowledged advantages of the critical distance construction over that of To the Editor, The Wireless Engineer the pentode were first pointed out in my articles SIR,-It was, of course, before the publication of in The Wireless World in 1935 and in The Wireless my own views on the subject (see " A Theory of Engineer (Bib. 2). Fluctuation Noise," paper read before the Wireless (7) In the above -mentioned original publication, Section of the I.E.E., 5th January, 1938) that I also described the principle of what is now termed Messrs. Percival and Horwood wrote their paper " reverse feed-back," and suggested its use with on " Background Noise Produced by Valves and critical distance valves. I show circuits for feeding Circuits " (Wireless Engineer, March, 1938, p. 328), back a portion of the voltage across an load and I suppose that in fact we must all have been to a or accelerating electrode for the working on the subject at about the same time. I purpose of reducing the effective anode impedance. am therefore rather concerned to find that the (8) No other critical distance valve or beam authors of this paper took for granted certain tetrode was, as far as I am aware, marketed until hypotheses which I consider are at least open to after the announcement of the by the Radio question, and certainly not valid without the most Corporation of America in April, 1936 (Bib. 3). careful qualification. This is some years after the research work in this i. Space -Charge Limited . The authors say country and over a year after the first commercial that it can be shown that, if the sole effect of space - use of the Hivac . The only feature of the charge were the formation of a potential barrier, the R.C.A. tube that did not previously exist in those effective temperature of the resistance would already made in England was that of making both be the same as in the absence of space -charge. It control grid and accelerating grid of the same pitch would be interesting to know how this result is and with the aligned. This feature is not, of obtained and what the temperature is, since my course, new to the valve art ; but unquestionably own conclusions (loc. cit.) were that in the absence reduces the screen current appreciably when applied of space -charge the temperature is indeterminate, to a critical distance valve. while in the presence of a potential barrier due to (9) There is also a curious and persistent em- space -charge the effective temperature is approxi- phasis in many quarters (Bib. 4) upon an endeavour mately, but not exactly, equal to half the cathode to explain the operation of the beam tetrode in temperature ; and this is without considering the terms of the usual effect, such as that hypothesis of fluctuations of the potential barrier. first described by Mr. E. W. B. Gill, in 1925 (Bib. 5). 2. . The authors say, " However, it has Certain properties of secondary radiation are studi- been shown by Williams that the effective tempera- ously ignored. It may readily be shown that an ture of the valve resistance may be thousands of electron space charge having the properties of those degrees absolute." This isolated quotation is, I known prior to the date of my original publication think, hardly fair, for it does not mention the fact in England is quite incapable of explaining the that Williams on the strength of such results, con- critical distance effect. It is not possible, for demns the whole hypothesis of a " thermal " reasons of space, to set out this matter more fully explanation of the fluctuation noise in both diodes if in this letter ; but it is hoped in the near future to and multi -electrode valves alike. But the publish a report prepared by the laboratories of this thermal theory is not indisputably established, does Company on the mechanism of the critical distance. it not seem dangerous to build upon it the super- For the same reason it is not possible to deal in structure of a hypothesis that " in some way an full with the history and origin of critical distance amplifying valve magnifies the effective tempera- valves. This is a matter which will probably be ture," without any indication of the mechanism, dealt with elsewhere ; but I hope that the above or other supporting evidence ? The present writer's information will be of interest to your readers. view is that the additional noise in a multi -electrode This country is entitled to any credit there may be valve is due to lack of complete space -charge for the production of the now widely used critical limitation, such as would render possible a thermal distance " beam tetrode." American vision and interpretation, in the region between anode and the American publicity have since popularised the preceding electrode (i.e. control grid in a , valve. It is a strange thing that British industry screen grid in a tetrode). so often refuses to progress until our cousins across 3. Screen Grid Valves. If the current reaching the the Atlantic have first gone ahead. anode is in any case random, as the present writer J. H. OWEN HARRIES, believes (loc. cit.), can sharing with the screen in a (Director, Harries Thermionics Limited). four -electrode valve make it " more random " ? London, S.W.i9. As seen at present, I fear that the method of measurement with anode joined to grid for A.C. BIBLIOGRAPHY will complicate rather than simplify theoretical interpretation, though it probably has advantages 1. Harries, Brit. Pat. 380429, App. date March 10th, 1931. Harries, Brit. Pat. 385968, App. date April 2nd, 1931. in the empirical classification of valves. 2. Harries, Wireless World, August 2nd, 1935. D. A. BELL. Harries, Wireless Engineer, April, 1936, Vol. 13, pp. 190-199. Harries, Wireless Engineer, February, 1937, Vol. 14, pp. 63-72 Great Baddow, .,. Schade, Electronics, Vol. 9, No. 4, April, 1938, pp. 18-21. Essex.

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