Vacuum tube

“ The Röntgen rays are certain invisible rays resembling, in many respects, rays of light, which are set free when a high pressure electric current is discharged through a vacuum tube. A vacuum tube is a glass tube from which all the air, down to one-millionth of an atmosphere, has been exhausted after the insertion of a platinum wire in either end of the tube for connection with the two poles of a battery or induction coil. ”

“ After the rarefaction has reached a certain point the discharge ceases to pass as a spark, and becomes apparently continuous. The arrangement in which this discharge is studied consists of a glass tube into which are sealed two platinum or, preferably, aluminium wires to serve as electrodes, and from which the air is removed to any required degree of exhaustion by an air-pump. Such an arrangement is usually called a vacuum-tube. ”

“ Of course, the micro-vacuum pump could be used in vacuum tube manufacture, in a pinch. Vacuum tube companies normally used large-volume pumps instead of the little super-efficient exhaustion pump that could take a few cubic centimeters down to a few millimicrons of mercury.
The electron microscope was a nice hunk of stuff, but the thing was not applicable to anything except research. ”

“ If we connect one side of the jars to the earth and the other side to the foot of one of the spirals or bobbins above described, we shall find perhaps that the vacuum tube at the other end is not rendered luminous. When, however, we adjust the inductance in the discharge circuit of the jar to a certain value to make the frequency of the condenser oscillations agree with the natural time period of the bobbin terminated by the vacuum tube, this latter at once lights up brilliantly. ”

“ One of the greatest difficulties experienced in getting these high vacua is that even when all the 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. ”

“ While the coil and tube are working, the terminal points of the induction coil are slowly brought together. If a spark passes between the points while they are six inches or more apart, the vacuum is too high. If no sparking takes place between the terminals till they are within three inches of each other, the tube is low. A good working spark gap distance is four and one-half inches. A soft, or low-vacuum, tube gives better definition than a hard, or high-vacuum, tube, as the rays pass less easily through dense substances and show greater differentiation of tissue. ”

“ The body of the tube is filled with a faint rosy light, which in many cases breaks up into a succession of bright and dark layers transverse to the direction of the discharge. The discharge in this case is called the stratified discharge. A vacuum-tube m which the exhaustion is such that the phenomena are those here described is often called a Geissler tube. As the exhaustion is raised still higher, the rosy light in the tube fades out, the non-luminous space around the negative electrode becomes very much greater, and the phenomena in the tube become exceedingly interesting. ”

“ A vacuum tube is one from which nearly all the air has been pumped, although we can never completely empty the tube. Crookes used such ingenious methods that he reduced the gas in his tubes until it was twenty million times thinner than the atmosphere. He then sent an electric discharge through, and got very remarkable results. The negative pole of the electric current (the "cathode") _gave off rays which faintly lit the molecules of the thin gas in the tube_, and caused a pretty fluorescence on the glass walls of the tube. ”

“ When a vacuum is produced of as low a pressure as a few hundredths of a micron (a micron is equal to 1/1000 of a millimeter of mercury in terms of barometric pressure) , the discharge is independent of the degree of vacuum, when the tube is used with appropriate space charge. The discharge then passing from cathode at constant temperature to anode, varies directly with the 3/2 power (square root of the cube) of the voltage imposed on the cathode. ”

“ The vacuum becomes higher as the tube is used, and after a while it becomes so high that the tube is practically useless, for the penetrating power of the rays is then so great that sharp contrasts between different substances, such as flesh and bone, cannot be obtained, and the resulting radiographs are flat and poor. The vacuum of a hard tube may be lowered temporarily by gently heating the tube, but this is not a very convenient or satisfactory process, and tubes are now made with special arrangements for lowering the vacuum when necessary. ”

“ The author had also observed that the conductivity of a vacuum tube without electrodes increased as the pressure diminished until a certain point was reached, and afterward diminished again, thus showing that the high resistance of a nearly perfect vacuum is in no way due to the presence of the electrodes. One peculiarity of the discharges was their local nature, the rings of light being much more sharply defined than was to be expected. They were also found to be most easily produced when the chain of molecules in the discharge were all of the same kind. ”

“ With a pump such as has been described there is seldom any advantage in fusing an extra connection to the vacuum tube so as to allow of a preliminary exhaustion by means of a water pump. About half an hour's pumping may possibly be saved by making use of a water pump.
The tube to be exhausted is washed and dried by careful heating over a Bunsen burner and by the passage of a current of air. The exhausting tube is then drawn down preparatory to sealing off, and the apparatus placed upon the pump. It is best held in position by a wooden clamp supported by a long retort stand. ”

“ The undoubted advantages to be derived from the use of vacuum tubes, especially in the field of wireless telephony where the output power must be modulated to conform to the intricate vibration pattern of the voice, has led to a demand for tubes capable of handling amounts of power comparable with those in use at the largest stations.
The the development of such tubes was of great importance was recognized by the engineer of the Bell Telephone System in the early days of the vacuum tube art. ”

“ The Operation of a Simple Vacuum Tube Receiving Set.--The way a simple vacuum tube detector receiving set works is like this: when the filament is heated to brilliancy it gives off electrons as previously described. Now when the electric waves impinge on the aerial wire they set up oscillations in it and these surge through the primary coil of the loose coupled tuning coil, a diagram of which is shown at B in Fig. ”

“ If one of Gassiot’s vacuum tubes be brought near to a powerful electrical machine, both while the machine is in motion and for some223 time after, flashes of light may be seen passing from the wire at one end of the tube to the other extremity, which flashes bear a great resemblance to the auroral rays. ”

“ A 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 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. ”

“ It consists of a glass cylinder, elongated into tubular form, and hermetically closed at the ends. When the tube is made, the air is exhausted as nearly as possible from it, and the ends are sealed over a vacuum as perfect as science is able to produce. Through the two ends, bits of platinum wire are passed at the time of sealing, so that they project a little within and without. The interior of the tube is thus a vacuum into which at the two ends platinum wires extend. Electrical communication with outside apparatus is thus supplied. ”

“ Experience shows that, where great exactness of manipulation cannot be obtained, it is necessary to allow only one train in each section of a tube, whether worked by vacuum or pressure. But where there is no intermediate station, and where the tube can be carefully worked, carriers may be allowed to follow one another at short intervals in a tube worked by vacuum, although it is not perfectly safe to do so in one worked by pressure. ”

“ Though three fall tubes are recommended, it must not be supposed that the pump will produce a Crooke's vacuum three times more rapidly than one fall tube. Until the mercury commences to hammer in the pump the three tubes will pump approximately three times faster than one tube, but as soon as the major portion of the air collected begins to come from the layer condensed on the glass surface of the tube to be exhausted and from the electrodes, the rate at which exhaustion will go on no longer depends entirely on the pump. ”

“ If the vacuum is poor, the life of the tube may be short or it may not start at all. To insure careful handling and safe delivery, Mercury-Arc Rectifier tubes are always shipped by express in the special box as they come from the factory. ”

“ Where the operator has not an adjustable tube it is imperative that he have at least two tubes, one high and one low. It is the contrasts which render the X-ray practical, and these contrasts are largely governed by the vacuum. In locating a metallic substance in the human body a high vacuum tube244 would be needed, that the bones and dense tissue be rendered more transparent. ”

“ A brilliant green light is often one of the signs of wrong connection, and particularly so when the little shadow on the chemical bulb is absent. Never run these or any other tubes backwards, but be sure the current is flowing in correct direction at first operation.
Other forms of Crookes tubes differ only in form, or are devoid of adjustment, and the connections of coil, tube, etc., are the same.
General Remarks.
A high vacuum gives greater penetrative power than a low vacuum. ”

“ As an illustration of this I may mention that the first experiment I tried with the discharge of a Leyden jar produced luminosity in the tube, but it was not until after six weeks' continuous experimenting that I was able to get a discharge in the exhausted tube which I was satisfied was due to what is ordinarily called electrodynamic action. It is advisable to have a clear idea of what we mean by electrostatic action. If, previous to the discharge of the jar, the primary coil is raised to a high potential, it will induce over the glass of the tube a distribution of electricity. ”

“ When a high vacuum is attained the efficacy of the pump depends chiefly on the way in which the drops fall on the head of the column. If the fall is too long the drops are apt to break up and allow the small bubble of air to escape up the tube, also any irregularity or dirt in the tube at this point makes it more easy for the bubbles of air to escape to the surface of the mercury. ”

“ Houskeeper adopted into the construction of the tube a remarkable type of vacuum seal which he had previously developed. These seals are made between glass and metal and can be amde in any desired size. They are capable of withstanding repeated heating and cooling over wide ranges of temperature, from that of liquid air to 350° C, without cracking and without impairment of their vacuum holding properties. ”

“ The most satisfactory method of testing that has been devised is the use of an orifice of proper size fixed to the hose end and measure the vacuum just inside of this orifice. In making such measurements care must be taken that the tube connecting to the vacuum gauge is not inserted in such a manner that the air velocity affects the reading of the vacuum gauge. ”

“ After a time the vacuum-bulb ceases to deliver bubbles of air; it and the attached tube are now to be heated with a moving Bunsen burner, when it will be found to furnish for 15 or 20 minutes a large quantity of bubbles mainly of vapor of water. After then production ceases the pump is righted and the exhaustion carried farther. In spite of a couple of careful experiments with the cathetometer I have not succeeded in measuring the vacuum in the vacuum bulb, but judge from indications, that is about as high as that obtained in an ordinary Geissler pump. ”

“ The striations are especially brilliant and steady when a Wehnelt cathode covered with hot lime is used and the discharge produced by a number of storage cells; by this means large currents can be sent through the tube, resulting in very brilliant striations. When the current is increased the positive column shortens, retreating backwards towards the anode, and may, by using very low currents, be reduced to a glow over the surface of the anode. The electric force in the positive column has been measured by many observers. It is small compared with the forces which exist in the dark space ”

“ The dirt is filtered out of the air or a mechanical trap is provided to catch and retain the dirt while the air is allowed to escape. Large-scale vacuum cleaners are used for cleaning city streets. The dirt is loosened by revolving brushes and is then drawn up by atmospheric pressure into a tank.
Another use of atmospheric pressure is to be found in the pneumatic tubes which convey mail from one post office to another. Air is exhausted from the tube and when a carrier is inserted which fits the tube closely the atmospheric pressure behind it forces the carrier along to its destination. ”