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VACUUM TUBE

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Originally appearing in Volume V27, Page 836 of the 1911 Encyclopedia Britannica.
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VACUUM See also:

TUBE . The phenomena associated with the pas-See also:sage of See also:electricity through gases at See also:low pressures have attracted the See also:attention of physicists ever since the invention of the frictional See also:electrical See also:machine first placed at their disposal a means of producing a more or less continuous flow of electricity through vessels from which the See also:air had been partially exhausted. In See also:recent years the importance of the subject in connexion with the theory of electricity has been fully realized; indeed, the See also:modern theory of electricity is based upon ideas which have been obtained from the study of the electric See also:discharge through gases. Most of the important principles deduced from these investigations are given in the See also:article See also:CONDUCTION, ELECTRIC (Through Gases); here we shall confine ourselves to the See also:consideration of the more striking features of the luminous phenomena observed when electricity passes through a luminous See also:gas. 28' Methods of producing the Discharge.—To send the current through the gas it is necessary to produce between electrodes in the gas a large difference of potential. Unless the electrodes are of the very See also:special type known as Wehnelt electrodes, this difference of potential is never less than 200 or 300 volts and may rise to almost any value, as it depends on the pressure of the gas and the See also:size of the tube. In very many cases by far the most convenient method of producing this difference of potential is by means of an See also:induction coil; there are some cases, however, when the induction coil is not suitable, the discharge from a coil being intermittent, so that at some times there is a large current going through the tube, while at others there is none at all, and certain kinds of measurement cannot be made under these conditions. Not only is the current intermittent, but it is See also:apt with the coil to be sometimes in one direction and sometimes in the opposite; there is a tendency to send a discharge through the tube not only when the current through the See also:primary is started but also when it is stopped. These discharges are in opposite directions, and though that produced by stopping the current is more intense than that due to starting it, the latter may be quite appreciable. The reversal of the current may be remedied by inserting in See also:series with the discharge tube a piece of apparatus known as a " rectifier " which allows a current to pass through it in one direction but not in the opposite. A See also:common type of rectifier is another tube containing gas at a low pressure and having one of its electrodes very large and the other very small; a current passes much more easily through such a tube from the small to the large electrode than in the opposite direction. Sometimes an air-break inserted in the See also:circuit with a point for one electrode and a disk for the other is sufficient to prevent the reversal of the current without the aid of any other rectifier.

There are cases, however, when the inevitable intermittence of the discharge produced by an induction coil is a fatal objection. When this is so, the potential difference may be produced by a See also:

battery of a large number of voltaic cells, of which the most convenient type, where more than a few milli-amperes of current are required, are small storage cells. As each of these cells only produces a potential difference of two volts, a very large number of cells are required when potential See also:differences of thousands of volts have to be produced, and the expense of this method becomes prohibitive. When continuous currents at these high potential differences are rquired, electrostatic induction See also:machines are most generally used. By means of Wimshurst machines, with many plates, or the more recent Wehrsen machines, considerable currents can be produced and maintained at a very See also:constant value. The exhaustion of the tubes can, by the aid of modern See also:mercury pumps, such as the Topler See also:pump or the very convenient automatic Gaede pump, be carried to such a point that the pressure of the residual gas is less than a millionth of the atmospheric pressure. For very high exhaustions, however, the best and quickest method is that introduced by See also:Sir See also:James See also:Dewar. In this method a tube containing small pieces of dense See also:charcoal (that made from the shells of coco-nuts does very well) is fused on to the tube to be exhausted. The preliminary exhaustion is done by means of a See also:water-pump which reduces the pressure to that due to a few millimetres of mercury and the charcoal strongly heated at this low pressure to drive off any gases it may have absorbed. The tube is then disconnected from the water-pump and the charcoal tube surrounded by liquid air; the See also:cold charcoal greedily absorbs most gases and removes them from the tube. In this way much higher exhaustions can be obtained than is possible by means of mercury pumps; it has the See also:advantage, too, of getting rid of the mercury vapour which is always See also:present when the exhaustion is produced by mercury pumps. Charcoal does not absorb much See also:helium even when cooled to the temperature of liquid air, so that the method fails in the See also:case of this gas; the absorption of See also:hydrogen, too, is slower than that of other gases.

Both helium and hydrogen are vigorously absorbed when the charcoal is cooled to the temperature of liquid hydrogen. When first the discharge is sent through an exhausted tube, a considerable amount of gas (chiefly hydrogen and See also:

carbon monoxide) is liberated from the electrodes and the walls of the tube, so thatto obtain permanent high vacua the exhaustion must be continued until the discharge has been going through the tube for a consider-able See also:time. One of the greatest difficulties experienced in getting these high vacua is that even when all the See also:joints are carefully made there may be very small holes in the tube through which the air is continually leaking from outside, and when the hole is very small it is sometimes very difficult to locate the leak. The writer has found that a method due to Goldstein is of the greatest service for this purpose. In this method one of the electrodes in the tube and one of the terminals of the induction coil are put to See also:earth, and the pressure of the gas in the tube is reduced so that a discharge would pass through the tube with a small potential difference. The point of an insulated See also:wire attached to the other terminal of the induction coil is then passed over the outside of the tube. When it comes to the hole, a very See also:bright See also:white spark may be seen passing through the See also:glass, and in this way the leak located. The See also:appearance of the discharge when the exhaustion is going on is a very See also:good indication as to whether there is any leakage in the tube or not. If the See also:colour of the discharge remains persistently red in spite of continued pumping, there is See also:pretty surely a leak in the tube, as the red colour is probably due to the continued influx of air into the tube. See also:Platinum is the only See also:metal which can be fused through the glass with any certainty that the contact between the glass and the metal will be See also:close enough to prevent air leaking into the tube. Platinum, however, when used as a See also:cathode at low pressures " sputters," and the walls of the tube get covered with a thin See also:deposit of the metal: to avoid this, the platinum is often fastened to a piece of See also:aluminium, which does not sputter nearly so much. See also:Tantalum is also said to possess this See also:property, and it has the advantage of being much less fusible than aluminium.

This sputtering depends to some extent on the See also:

kind of gases present in the tube, as in monatomic gases, such as mercury vapour, even aluminium sputters badly. Electrodeless Tubes.—As some gases, such as See also:chlorine and See also:bromine, attack all metals, it is impossible to use metallic electrodes when the discharge through these gases has to be investigated. In these cases " electrodeless " tubes are some-times used. These are of two kinds. The more usual one is when See also:tin-See also:foil is placed at the ends of the tube on the outside, and the terminals of the induction coil connected with these pieces of foil; the glass under the foil virtually acts as an electrode. A more interesting See also:form of the electrodeless discharge is what ' is known as the " See also:ring " discharge. The tube in this case is placed inside a wire solenoid which forms a See also:part of a circuit connecting the outside coatings of two See also:Leyden jars, the inside coatings of these jars being connected with the terminals of an induction coil or electric al machine; the jars are charged up by the machine, and are discharged when See also:sparks pass between its terminals. As the discharge of the jars is oscillatory (see ELECTRIC WAVES), electric currents See also:surge through the solenoid surrounding the discharge tube, and these currents See also:reverse their direction hundreds of thousands of times per second. We may compare the solenoid with the primary coil of an induction coil, and the exhausted bulb with the secondary; the rapidly alternating currents in the primary induce currents in the secondary which show themselves as a luminous ring inside the tube. Very bright discharges may be obtained in this way, and the method is especially suitable for spectroscopic purposes (see Phil. Mag. [51, 32, pp.

321, 445). Appearance of the Discharge in Vacuum Tubes.—Fig. rs b of the article CONDUCTION, ELECTRIC (Through Gases) represents the appearance of the discharge when the pressure in the tube is comparable with that due to a millimetre of mercury and for a particular intensity of current. With See also:

variations in the pressure or the current some of these features may disappear or be modified. Beginning at the negative electrode k, we meet with the following phenomena: A velvety glow runs, often in irregular patches, over the See also:surface of the cathode; this glow is often called the first negative layer. The spectrum of this layer is a bright See also:line spectrum, and See also:Stark has shown that it shows the Doppler effect due to the rapid See also:motion of the luminous particles towards the cathode. Next to this there is a comparatively dark region known as the " See also:Crookes' dark space," or the second negative layer. The luminous boundary of this dark space is approximately such as would be got by tracing the See also:locus of the extremities of normals of constant length See also:drawn from the negative electrode; thus if the electrode is a disk, the luminous boundary of the dark See also:sphere is nearly See also:plain over a part of its surface as in fig. t, while if the electrode is a ring of wire (fig. 2) the luminous boundary resembles that 0 etn shown in fig. 17 of the article CONDUCTION, ELECTRIC (Through Gases). The length of the dark space depends on the pressure of the gas and on the intensity of the current passing through it. The width of the dark space increases as the pressure diminishes, and may, according to the experiments of See also:Aston (See also:Pro. See also:Roy.

See also:

Soc. 79, p. 81), be represented with considerable accuracy by the expression a+b/p or a~ cX, where a, b, c are constants, p the pressure and X the mean See also:free path of a corpuscle through the gas. The thickness of the dark space is larger than this free path; for hydrogen, for example, the value of c is about 4. Vi See also:hen the current is so large that the whole of the cathode is covered with glow the width of the dark space depends upon the current decreasing as the current increases. In helium and hydrogen Aston (Pro. Roy. Soc. 8o A., p. 45) has detected the existence of another thin dark space quite close to the cathode whose thickness is See also:independent of the pressure. The farther boundary of the Crookes dark space is luminous and is known as the negative glow or the third negative layer. Until the current gets so large that the glow next the cathode covers the whole of its surface the potential difference between the cathode and the negative glow is independent of the pressure of the gas and the current passing through it; it depends only on the kind of gas and the metal of which the cathode is made.

This difference of potential is known as the cathode fall of potential; the values of it in volts for some gases and electrodes as determined by Mey (Verb. deuts. Phys. Ges., 1903, V. p. 72) are given in the table.

End of Article: VACUUM TUBE

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