Wireless World

Evening Post, Volume CXII, Issue 116, 12 November 1931, Page 22

Wireless World

By "Grid Bias"

-500 KILOWATTS THE WORLD'S BIGGEST VALVE - Accompanying this article is a photograph of what is by far the biggest radio valve ever made. It has been made by the Metropolitan Vickers Co. to the order of the British Post Office, for use at the great Rugby wireless station, and is capable of handling up to 500 kilowatts of power—an output about twenty times as great as that of any of its predecessors. As is well known, nearly all valves are contained in a glass or silicon container to maintain the required vacuum; and under operating, conditions the vacuum in high-power valves cannot be long maintained. Moreover, when a valve fails, its replacement >* costly. For many years the ideal of a valve whose vacuum would not become poorer with age, in which the great difficulty of sealing off the electrodes under vacuum in the process of manufacture would be avoided and in which the filament could be replaced or other repairs executed, and the valve rapidly and easily again placed in operation, has been the object of much investigation. Continuously evacuated valves , have previously been operated successfully under laboratory conditions, the vacuum being maintained by means of the mercury vapour pump. This process, however, was rendered commercially impracticable by the fact that the mercury vapour prevented the necessary

Ar'ree of vacuum from being maintained" unless recourse was had. to the expensive and unwieldy process of cooling with liquid air. Herein lies a. romantic story of scientific achievement and of a discovery which has been developed by close co-operation between • scientist;, en-, gineer, and ■ artisan to its practical utilisation in the world's largest wireless valve. The discovery leading to this achievement emanated from the research laboratories of the Metropoli-tan-Vickers Company, where some three years ago experiments in no way connected with wireless communication or thermionic valves, and indeed not expected to be of any industrial consequence, led to the production of oil distillates having remarkable properties. They could be boiled at a fairly low pressure without decomposition and yet at room temperature their rate of evaporation was so small that they could be placed inside a wireless valve' without impairing the vacuum. Such a material has almost _as much appeal to the vacuum physicist of today as had the philosopher's stone to the early alchemist. It was quickly realised that here was the ideal liquid to replace the mer-

cury of the vapour pump, as due to itslow volatility cooling could now be effected with water and the cumbersome and expensive liquid air process eliminated. In 1930 a valve designed for an input of approximately 25 kilowatts was constructed, incorporating the new development, and proved so satisfactory in operation that the G.P.O. entrusted the company with the design and. manufacture of a 500 kilowatt valve. Physically this huge new valve stands 10ft high and is 14in in diameter. Complete it weighs over a ton, and is built on a welded steel bedplate Bft long by 3ft wide. Its water-cooled steel anode weighs Scwt, and is equipped with hydraulic jacks to facilitate demounting and assembly. Its filament current is approximately 500 amperes, or about 5000 times that of a receiving valve, and its filament emission 160 amperes, representing an electron flow of 300,000,000,000,000,000 (three hundred thousand billion) electrons per second. It will operate the main transmitter at Kugby, replacing a bank of 50 ordinary high-power valves. No glass is used in its construction, but a robust combination of steel and porcelain. ■ As a result of the continuous evacuation, and in sharp contrast with valves of the permanently sealed type, this new valvo improves vith age, any gas evolved during its operation and which in the sealed valve would inevitably impair the vacuum, being rapidly removed by the pumps. Thus an exceedingly high vacuum is maintained even at the highest power inputs. The pumps, with the exception of a simple type of primary extractor, have no moving parts, and mechanical ' trouble is therefore a very remote pos-

sibility. • The valve ■is completely demountable. In the event of "filament or other repairs being necessary, it may be rapidly taken to pieces, the repairs executed .with ordinary engineering tools, and the whole re-assembled and placed back into operation in the course- of' a few hours. These new developments have made possible the manufacture of valves capable of dealing with outputs considerably higher 'than 500 kilowatts, and demountable continuously evacuated valves of many times this power —in itself several times that of what was previously the world's largest—■ are in no sense a visionary prospect. It is fitting that at a time when the homage of the scientific world is being paid to the memory of Michael Faraday, who 100.years ago laid the foundation of electrical engineering by his momentous discovery and exposition of the principle of electro-magnetic induction, British inventive genius should add yet another outstanding discovery to its record of . achievement. The G.P.O. regarded this achievement as of such outstanding importance and merit that it exhibited the valve at the Albert Hall on the occasion of the Faraday centenary celebrations.