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Television has been largely responsible for the rapid development of the cathode-ray tube, which is now a comparatively robust and reliable piece of apparatus with a reasonable expectation of life, and is consequently finding applications in many new fields.
The cathode-ray tube is being used to an increasing extent in industry owing to the ease with which it can be made to give visual indication of various phenomena. The simple analysis of the wave-form of currents or voltages is now almost the least important application.
Let us examine in detail some of the applications. The general principles will be familiar to readers. By an assembly be familiar to readers comprising a cathode, grid and one or more anodes a beam of electrons is produced which is projected down the tube on to a fluorescent screen where it impinges with a sharp focus, causing a small spot to appear. Deflector plates located on each side of the beam, shortly after it first emerges from the gun or anode system, cause the beam to deviate in its travel and hence move the spot. either horizontally across the screen or vertically up and down. Similar movement of the spot can be obtained by passing current through coils located externally to the tube, and in some circumstances this magnetic deflection is convenient.
It is customary to cause the spot to move in a horizontal direction by the application of voltage from a device called a time base. It is interesting to think what this name really implies. It means simply that movement of the spot is proportional to the time which has elapsed since it started to move. If we place on the vertical deflector plates a voltage from an oscillator or similar source of AC the vertical position of the spot at each instant will be proportional to the manner in which the particular voltage is varying. At the same time the spot is being caused to move horizontally at a rate proportional to the time interval so that the variation of the voltage under examination is spread out on a time base. As we know, this results in the familiar wave-form, which is quite smooth and regular if the wave-form is pure but contains all sorts of humps and irregularities if the wave-form is distorted, or if made up of more than one component frequency.
Fig. 1. - Illustrating the principle of the marine echo sounder Inset: Record of depth as it appears on the tube.
Apart from the simple examination of wave-form, there are quite a number of other applications which require a time base. One unusual application is that of echo sounding or depth recording. The art of measuring depth electrically has been brought to a high state of perfection of recent years. The procedure is to generate a sound wave by a diaphragm located in the hull of the ship. This sound wave travels down to the bottom of the sea bed, is reflected back again, and is picked up on a microphone. Since the velocity with which sound travels through water is known, an accurate observation of the time elapsing between the start of the sound and its reception by the microphone gives an immediate indication of the distance which the sound has travelled down to the bottom of the sea bed and back.
It is clear that any equipment of this sort must not involve elaborate calculation, for it has to be used by quite inexperienced operators who will require merely a clear indication of depth without any juggling. The cathode-ray tube offers an immediate solution to this problem. By a suitable time base the spot is caused to move relatively slowly across the screen. It starts its travel at the instant when the sound pulse first leaves the transmitter in the ship's hull. When the reflected sound is picked up by the micro-phone it causes a deflection on the vertical plates which results in a kink in the wave. The distance of this kink from the starting point indicates the depth, and by mounting the tube behind a transparent screen having a superimposed scale calibrated in fathoms it is possible to see at a glance the depth of the ocean bed over which the ship is moving at the time.
Heart Action Made Visible
Another application requiring a time base is the cardiograph - an electro-medical instrument for observing the action of a patient's heart. The heart's action generates electrical voltages between the extremities of the limbs, and the wave-form of these voltages is a useful clue to the condition of the patient's heart.
Since the-heart beats occur 70-80 times a minute under normal conditions, a slow-moving time base is again required which will deflect the spot across the screen once every two or three seconds - sufficient to enable two or three successive heart beats to be observed. This particular application also involves the use of a screen having a long afterglow - a point which will be discussed later in this article Other types of indication, however are often required. For example, consider an audio-frequency response curve of an amplifier. Here the vertical movement of the spot would again be proportional to the output voltage, but in this case it has to be related not to a time base but to the frequency of the current at each instant.
One way of doing this is to use a beat-frequency oscillator so designed that it will cover the whole frequency range required in one complete rotation of a suitable shaft. On the same shaft is mounted a potentiometer from which voltage is tapped off and taken to the horizontal deflector plates of the tube. Thus the voltage actually applied to the plates is proportional to the rotation of the shaft, which again is proportional to the frequency, and hence the tube can be made to plot the actual low-frequency response curve. It will do this at each revolution of the shaft, and if the shaft is revolving fairly rapidly the successive traces will all lie on top of one another and will give the impression of a continuous and steady curve. The effect of altering various constants in the circuit can be observed at once, and the curve can, in fact, be adjusted to acquire any desired shape in a very short time without the necessity for laborious measurements.
Aid to Circuit Alignment
Fig. 2. - Electrical frequency base as used with cathode-ray tubes.
The same idea can be applied to high frequencies, where one often requires to know the resonance curve of the circuit or group of circuits. This is particularly the case with IF transformers, for the modern bandpass transformer is difficult to line up (even with a valve voltmeter), so that the peaks are exactly as they should be.
By having a similar arrangement whereby the rotating shaft alters a small capacitor in parallel with the main tuning capacitor of a radio-frequency oscillator it is possible to vary the frequency within, say, plus or minus 20 kHz of the resonant point. As the frequency varies over this range, the output from the IF transformer will rise to its maximum and fall off again, and if the cathode-ray spot is given a horizontal deflection proportional to the frequency by the same method as before the resonance curve will build up on the tube. Simple adjustment of the trimmers is then sufficient to pull the curve into any desired shape, and it is quite easy to adjust them to be of the same height and symmetrically disposed.
It is worth noting that mechanical methods are not essential for producing the variation of frequency required. It is usually practicable to cause a suitable variation in some part of the circuit by electrical means pure and simple. For example, the input capacity of a valve is dependent upon the effective amplification which the valve is delivering at the time. Now the amplification of a valve can easily be controlled by using a variable-μ type and altering the bias. Therefore, by connecting across the oscillating circuit a passenger valve having a suitable anode load and varying the bias on this valve the effective capacity in parallel with the main tuning circuit can be varied within certain limits. The variation of bias on the grid may be accomplished by applying suitable alternating potential from the mains, and the same potential may be used to produce the deflection on the horizontal deflector plates, thus producing a very simple electrical form of frequency base.
There are numerous other forms of frequency base and other ways of achieving the desired results, but we must pass on to yet another form of application, that requiring a voltage base.
A good example of another type of base - the voltage base - is provided by a valve characteristic where we desire to know how the anode current varies according to the voltage on either the grid or the anode. From what has been said already it will be clear that this is comparatively simple to arrange. We require a source of varying voltage which is applied to the grid or anode of the valve, and at the same time to the horizontal deflector plates, so that as the voltage on the valve varies the spot moves from side, to side on the cathode-ray tube.
Then by arranging that the vertical deflection of the spot is proportional to the anode current the tube will plot the valve characteristics. The vertical deflection may be obtained magnetically by passing the anode current through suitable coils or by including a small resistance in the anode circuit and using the voltage developed across this resistance, if necessary through an amplifying valve, as described in these columns (May 8, 1936).
Here, again, the usefulness of the device lies in the fact that any alteration to any of the constants in the circuit causes an immediate alteration in the characteristic which can be seen at once. I have actually turned the characteristic of a screen grid valve inside out by adjustment of the anode and grid potentials!
There is one other application of voltage bases which may be mentioned particularly, as it depends on a most interesting property of cathode-ray tubes: that of afterglow. The spot of light on the screen is caused by the fluorescence of the material under the influence of the electrons. Now, in the ordinary course we require that the luminosity shall cease immediately the electron beam is removed. Otherwise, if the beam is moving rapidly across the screen the spot will leave behind it a little tail like a comet, due to a slight persistence of the luminosity.
This persistence is called the afterglow, and considerable research has been undertaken to reduce it to a minimum. With a normal tube it only exists for a small fraction of a second and is thus barely noticeable, but in television and similar applications the spot has to move at such an extremely rapid rate that an afterglow of a few micro-seconds is sufficient to cause trouble.
On the other hand, there are occasions when an afterglow of this sort is useful, and by working in the opposite direction modern research has now produced tubes having an afterglow of 10-15 seconds and more. This means that if the spot moves across any part of the screen it leaves a trace which will remain glowing, even though the spot has long since ceased to be anywhere near, and may, in fact, be executing a movement in some other part of the screen.
Fig. 3. - Circuit for cathode-ray writing.
So pronounced has this afterglow been made that it is possible to make the spot write words on the screen and the trace is visible for a sufficiently long time for the words to be read quite easily. A pencil is used which operates two potentiometers in the form shown in Fig. 3, so that the horizontal and vertical location of the pencil point relative to its frame are immediately translated into voltages which are applied to the deflector plates of the tube and cause the spot to take up an exactly equivalent position on the cathode-ray screen. Then as the pencil is moved the spot on the screen moves in conformity and will write words and even messages. It is possible for something like ten words to appear on the screen at once before the glow dies away, i.e., as the eleventh word is being written the first word is just becoming indistinguishable.
Enough has been said to show that possibilities of the cathode-ray tube are far greater than anyone would have imagined a few short years ago. Even now several quite interesting applications have had to be omitted for lack of space. The ordinary user can take comfort in the thought that the more cathode-ray tubes are used in industry the cheaper they will become, until we can look forward to the time when the tube will cost little more than a super-power valve!
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