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The Twenty-Kilowatt Tube

Dr Irving Langmuir: Popular Wireless: October 7, 1922.
    
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The three-electrode vacuum tube, which first appeared as the De Forest audion, is now universally used not only for the receiving of radio messages, but is coming into more widespread use in connection with the transmission of such messages. The original De Forest audion did not have a particularly high vacuum and, because of the ionisation of the residual gas, could not be operated at more than 30 or 40 volts, or at more than a few milliamperes of current.

First Knowledge

Several years ago, in connection with a study of the Edison effect in incandescent lamps, I noted that, in lamps with a very good vacuum, the Edison effect was nearly absent. In other words, although there was a difference of 11 volts between the two ends of the filament, very little electron current flowed across the vacuum space between. In the presence of a little gas, however, such big currents were obtained that the currents might lead to the formation of an arc, with resulting destruction of the lamp.

At that time it was not at all understood why these currents should become smaller as the vacuum was improved, and a great many scientists believed that if a perfect vacuum could be made no current at all would flow across it. Although the Edison effect in well made lamps thus caused no difficulty in their manufacture, for it practically did not exist, yet it was a point of very great scientific interest to learn why their currents were so small in a good vacuum.

Basis of the Tube

It was in connection with these studies that we discovered a 'space charge effect'. We then understood it that in a high vacuum the electrons got in each others way so that the electrons that, had already left the filament repelled, because of their negative charge, the electrons which followed and tended to drive them back into the hot filament which emitted them. In the presence of gas this effect did net exist, because the gas formed both positive and negative ions, and the accumulation of the slowly moving ions in the space neutralised the effect of the negative electrons

As the result of these studies it gradually became clear how it would be possible to construct vacuum tubes which would operate at high voltages and at high currents. One of the early applications of this new knowledge was made by Dr W D Coolidge, who utilised this in the development of the Coolidge X-ray tube, an X-ray tube which has gradually displaced practically all oflthe older, so-called gas tubes.

Another application was. found in the kenotron and pliotron. The kenotron is a vacuum tube rectifier, having two electrodes like the Fleming valve, but capable of operating up to voltages of several thousand volts and with currents comparable with an ampere or more. Tubes of this. kind have found application for smoke precipitation, for various electrical testing devices, and in connection with the regulation of the electric generators used for the radio transmitting outfits on aeroplanes during the war. The development o 'the kenotron into a thoroughly practical device for these purposes is largely the result of the work of Dr Saul Dushman.

The pliotron bears about the same relation to the De Forest audion that the kenotron does to the Fleming valve. It is a device which contains three electrodes, namely a filament, grid, and plate, like the audion, but it is capable of being operated at high voltages and currents, so that considerable amounts of power may be controlled. Tubes of this sort are now finding widespread application for transmitting radio messages, particularly for radio telephony. The ordinary radio telephone outfit used for broadcasting generates from 0.5 to 5 kW of high frequency power, which is used to feed the antenna.

The design and construction of tubes of this type has been carried out principally by W C White.

Its Construction

It has long been realised that, following out the principles made use of in the smaller tubes, it would ultimately be possible to construct tubes of large power. There have been many difficulties to overcome, however. After years of work in the Research Laboratory by W C White and H J Nolte, they have succeeded in designing and perfecting pliotrons which are capable of generating about 20 kW of high frequency current. In principle, these tubes resemble the smaller tubes which are now usually called radiotrons, in that they also have three electrodes. These large tubes are used in circuits much like those used by amateurs when they cause the tube to generate oscillations. In the construction, however, there are many differences.

The 20 kW tube has a very large, rugged filament many times the diameter and length of the ordinary radiotron. The grid is in cylindrical form and surrounds the filament, and the plate is a metallic cylinder about 1.5 inches in diameter and 8 inches long, which sealed directly to a glass tube, through which pass the leads carrying current to the filament and grid.

The Future

Thus the plate, instead of being inside the tube, as in ordinary radiotrons forms a part of the outside wall of the tube. In order to dissipate the relatively large amount of energy liberated, at the plate, the plate is water-cooled, which is rendered particularly easy by the fact that part of its surface forms a part of the wall of the tube.

These 20-kW tubes are ordinarily operated with about 20,000 volts DC, which is obtained from ordinary 60-cycle alternating current by rectification, using two or more kenotrons, together with large capacitors for smoothing out the rectified alternating current.

These 20-kW tubes are ordinarily operated with about 20,000 volts DC, which is obtained from ordinary 60-cycle alternating current by rectification, using two or more kenotrons, together with large capacitors for smoothing out the rectified alternating current.

A bank of ten tubes of this kind operated in parallel is capable of generating 200 kW of power, which is about all that is required for most trans-oceanic radio communication. It is probable that outfits of this kind will displace the larger and more expensive alternators, the most successful type of which has been the Alexanderson alternator.

The 20 kW tube merely marks one stage in the development of still larger tubes. It will undoubtedly be possible, when the need arises and when the necessary development work has been completed, to construct tubes of many hundreds, or even thousands, of kilowatts. Such devices will probably be used not merely for radio purposes, but may ultimately play, an important part in such problems as the electrification of railroads and the transmission of power to long distances by means of direct current.

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