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Vapor Converter and Allied Apparatus
By P. H. THOMAS
ance to current flow which is found at the negative-called the “negative electrode resistance"—is so great under ordinary conditions that many thousand volts are required to overcome it. Strange to say, all that is necessary to practically elimi
IN the August, 1904, number of The
TECHNICAL WORLD (p. 666), the 1 Hewitt Mercury Vapor Lamp was
described at some length. It was then made clear that the invention of Mr. Peter Cooper Hewitt was rather the discovery of a new group of phenomena in electrical physics than the invention of a new lamp. This lamp is merely one of the results of the discovery of the new principle.* The Mercury Vapor Converter is another. Principle of Negative Electrode
Resistance This new principle, which concerns the passage of electricity through a vacuum tube or bulb, may be briefly explained. For electric current to pass through a vacuum, two electrodes are required, one to lead it in, and the other to lead it out. These electrodes are merely electrical conductors exposed in the vacuum space. The electrode by which the current enters is called the anode or positive; and that by which it leaves, the cathode or negative. The positive and negative electrodes must evidently be connected, respectively, to the positive and negative supply lines. It is found by experiment, that, except under certain conditions, a very great resistance to the passage of current exists at the surface of the negative electrode of a vacuum tube; and that to operate all types of such apparatus (except under these particular conditions), this great resistance has to be continuously overcome by the application of a high electromotive force.
The necessary high electromotive force has usually been obtained from an in. duction coil. Little or no resistance to the passage of current is found at the anode, however. This surprising resist
*NOTE.It should be here stated that, unknown to Mr. Hewitt, Aarons of Germany had previously operated some small glass vacuum tubes, utilizing the same principle. The Aarons device, however, was not suitable for illumination, and was never further developed.
potential or other means before the negative electrode resistance will disappear. In other words, whenever the initial negative electrode resistance is broken down, current will continue to flow, meeting only slight resistance until the current stops of itself, when the original initial negative electrode resistance will re
Mercury Vapor Lamp in recent issue of The TECHNICAL World, above referred to.)
2. From the fact that an initial extra voltage is required to produce the state of low negative electrode resistance, a vapor converter can be constructed which will change alternating current into direct current.
3. Owing to this same property, the apparatus can be used as a circuit-breaker for alternating currents.
4. Owing to the combination of the two properties described above (1 and 2), the apparatus can be advantageously used as a discharge gap, sometimes called an interrupter, to replace the air spark gap commonly employed for wireless telegraphy and other purposes.
Fig. 2. VAPOR CONVERTER WITH
Two POSITIVE ELECTRODES.
establish itself. The reason that older types of vacuum apparatus do not act in the same manner as Mr. Hewitt's tube, is that the supply of current is intermittent in its nature, and that the negative electrode resistance is being continuously re
ished by the interruption of the current. Further, it is found that a liquid negative, such as mercury, is the only one that will operate continuously with out deterioration. The old types of apparatus practically all use solid negatives.
Mr. Hewitt discovered that current passing through a vacuum charged with mercury vapor causes this vapor to emit an intense light, with a comparatively small expenditure of energy. His discovery of how the negative electrode resistance can be eliminated, allowed him to make economical use of this very efficient and new method of producing ht. Hence his miercury vapor lamp.
Applications The great principle of the "negative electrode resistance” has thus been briefly explained. Various practical applications can be made of it, which may be summarized as follows:
1. Owing to the low operating "negative electrode resistance," a vacuum electric lamp
ricam can be economically used. (See description of
Direct from Alternating Current To make clear the usefulness of the vapor converter, it will be necessary to state briefly the characteristics of alternating as distinguished from direct currents.
An electric circuit carries direct current when electricity passes in one direction continuously through the circuit. A circuit carries alternating current when at one moment electricity passes in one directic:throughout the whole circuit, and at a later instant of time in the opposite direction throughout the whole circuit ; and so on. Only direct current is suitable for charging storage batteries. Direct current is much more suitable than alternating for operating a great many ciasses of motors, such as elevator and boisting motors, and, up to the present time, motors for street-car or railway work. It is found, however, that alternating current is very much more ecocomical than direct current for transmitting and distributing electrical power. Consequently, since only direct current is suitable for many purposes, it will often be advantageous to be able to obtuin direct current from alternating. This result can be accomplished, at the present time, by means of certain electrical apparatus called the “rotary converter,” which is very similar to an electric generator or motor, but which is large and expensive, requires expert handling, and is not particularly efficient. It is the function of the vapor converter to provide a more satisfactory method of obtaining direct current when only an alternating-current service is available.
In the diagram a. Fig. 3, are shown the variations of ordinary alternating
3, at point A, where we shall assume the alternating current begins, it will be observed that the value of current begins to decrease, until finally a zero point is reached at B. At this time, of course, current ceases to flow. Meanwhile the alternating electromotive force or voltage is reversed, and begins to increase in strength. This reversal of direction makes the other electrode of the apparatus the negative; and, if we provide no means of overcoming its initial negative electrode resistance, no current can flow. The same condition will exist also when the alternating force has again become zero, and flows in its original direction, as indicated at the point C (Fig. 3). If the electrode resistance were broken down again at this point C, of course, the lamp would start again,
current or alternating electromotive force. Consider that in this figure the distance of the curved line above or be low the horizontal line indicates the strength of the current or voltage at any particular instant of time; and that, the further we go to the right on the horizontal line, the later the instant of time indicated. When the curve is above the horizontal line, the current in the circuit, or the electromotive force, is in one direction; and when the curve is below the horizontal line, it is in the other direction. It will be noticed that in changing from one direction to the other, the value of the current or voltage gradually decreases to zero, and then increases in the other direction up to a maximum, when it again decreases to zero, and returns to its original direction; and so on.
Let us now consider the Hewitt apparatus. This may be briefly described as a large glass bulb or globe, with a small puddle of mercury at the bottom constituting the negative electrode, and, supported from the top part of the bulb, two or more electrodes of iron or other material. These electrodes are of various shapes, and usually not over one or two inches in size. Platinum lead wires run from all the electrodes to the outside line wires, through the glass, which makes a perfectly air-tight joint with this metal. The bulb is exhausted as perfectly as possible of all gases except mercury vapor. A converter of this type is shown in Fig. 1. In some cases it is desirable to make the electrodes all of mercury. Such a converter, with four positives and a negative, is shown in Fig. 8.
On account of the nature of the negative electrode resistance, if we connect a Hewitt mercury vapor apparatus across suitable direct-current mains, and start it by breaking down the negative electrode resistance, current will flow steadily until it is stopped by means external to the apparatus, this being the proper operation of the apparatus on direct current for giving light.
Suppose, however, we suddenly change the direct current to alternating current. At the instant of change, nothing new occurs. There is no difference in the operation of the apparatus, since we shall assume there is at the instant no change in the current. Then, referring to Fig.
and would run until the current again became zero at the point D. It is thus evident that this apparatus cannot run continuously of itself on alternating current.
Suppose, however, instead of the ordinary lamp, which has but two electrodes, we use an apparatus like that shown in Fig. 2, which has two electrodes at one end, and one at the other. Suppose we pass direct current between electrodes i and 3 in such a direction that 3 is the negative or cathode. Then, as in the mercury vapor lamp, current will continue to flow between these electrodes; and the negative electrode resistance of electrode 3 will be practically
Take another case. Suppose we have a second alternating current exactly opposite to the first, and suppose we have a fourth terminal in Fig. 2 similar to Fig. 1, and that the new alternating current is sent from the new electrode to 3. The relation of the two alternating electromotive forces is shown in Fig. 3. It is evident that the second alternating current, like the first, will pass only those alternations which are in the right direction, and that the others will be suppressed. But the former come just at the times when the first alternating current is not acting, so that the gaps of direct current left by the first are filled in by the second, and the current flowing in the negative is much more nearly steady than in the first case. This current is shown at b in Fig. 5.
Should the direct current from 1 to 3 be suddenly stopped, alternating current would continue to flow from 2 or from the extra electrode, whichever happened to be then operating, until the first zero point was reached, when, since both alternating electromotive forces are zero at the same time, all current would cease and the apparatus refuse to operate.
Now, suppose that the vapor apparatus shown in Fig. I be operated without direct current, and suppose that three different alternating currents be applied, each one between the negative and one of the three positive electrodes. These alternating electromotive forces are to be considered as three-phase electromotive forces—that is, each one lags 1-3 of a complete double alternation behind the one before it, as shown in c (Fig. 3). Suppose the electromotive force E in this figure to be passing a current from its proper positive electrode to the negative in the three-phase converter, shown in Fig. 1. As this electromotive force becomes less and less until the point M is reached, the current through the converter would become less; but at this point the electromotive force F is seen to have risen to the same value in the same direction to which the electromotive force E has fallen ; and thus, evidently, the electromotive force F, becoming greater than E, takes the current from E, and causes the current to increase in value again, following the curve F of the figure until the point N is reached, where
Fig. 4. CONVERTER For Single-Phase
the direct, has no negative electrode resistance, on account of direct current which flows to it from electrode 1. As the alternating electromotive force changes to the opposite direction, it at tempts to flow from 3 to 2; but, since in this case electrode 2 is its negative, and since its initial electrode resistance is not broken down, the reverse alternation of the alternating current cannot start. When the alternating electromotive force returns to its first direction again, however, 3 is its negative, and current will flow as before.
Thus only those alternations of the supply which have the direction from 2 to 3 can pass, and we have sticceeded in getting one kind of direct current from an alternating source. The intermittent character of this current is shown at a in Fig. 5. This is perhaps the simplest form of converter. The direct current, however, though all in the same direction, is intermittent, and not well adapted for some kinds of work.
the electromotive force G in turn takes the current away from F, and carries it to the point P, at which the electromotive force E, having gone through a complete half-cycle, is ready again to take up the current from G and carry it to the point Q; and so on indefinitely. With this type of circuit there is evidently no time at which the current through the negative electrode becomes zero; and, consequently, the converter continues to operate indefinitely without the necessity of supplying direct current.
Such three-phase electromotive forces as are here described can be obtained from any three-phase circuit in which the neutral point is available. This is the type of converter first shown by Mr. Hewitt. It will readily be seen that in this case the direct current in the negative electrode—which is the current that is available for useful work-has a much steadier value than would be obtained by the apparatus of Fig. 2. Similarly, with an apparatus having still another positive electrode and four alternatingcurrent electromotive forces, we should again have a converter in which the elec
figure it is assumed that there is no choke coil to smooth out the variations of the current; whenever such choke coils are used, the current is materially steadied.
On many alternating-current circuits, however, only one voltage is available, and there is no direct current for bridging the zero points as was found necessary in Fig. 2. For these cases, it is very desirable to have some means of operating a vapor converter. This can be accomplished by the apparatus shown in Fig. 4, which is the same as that of Fig. 2, except for the connection to the supply circuit, and except for the choke coil placed in lead from the negative electrode.
Referring to the figure, suppose that the voltage between the electrode P and the middle point R of the supply transformer be supplying current through electrode 1; this current is delayed behind the electromotive force, since it has to flow through the choke coil. This means that when the electromotive force from I has dropped to zero, the current has not yet reached zero, since it lags behind. The converter does not go out at this instant. It would, however, go out a little later, when the current finally did reach the zero point; but the electromotive force between the terminals S and R, which is exactly opposite to that between P and R, is trying to force current from electrode 3 to electrode 1, and, on account of the lag of the current from 1, it will pick up this current before it be
tromotive forces so overlapped as to require no direct current for bridging the zero points of the current. Such alternating electromotive forces can be obtained from any two-phase or 14-phase circuit where the neutral point is available. In this apparatus the current in the negative is still more nearly steady than in the case of the three-phase converter.
The currents from the different types of alternating-current circuit are compared as to steadiness in Fig. 5. In this
rest of the alternation, until this electromotive force in turn approaches zero. Again, since the current is lagging behind, the electromotive force between P and R will pick it up from R and S before it actually becomes zero, preventing the converter from stopping ; and so on. The simple introduction of the choke coil enables the apparatus of Fig. 2, when supplied by the proper electromotive force from a single-phase circuit, to operate indefinitely without the use of auxiliary direct current. This is one of the most important applications of the converter. On account of the choking power of the coil, this arrangement gives quite a steady current; but, as in most of the cases above, it requires the neutral point of the alternating-current supply.