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Vapor Converter and Allied Apparatus


Chief Electrician, Cooper Hewitt Electric Company


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

N the August, 1904, number of The
TECHNICAL WORLD (p. 666), the
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 induction coil. Little or no resistance to the passage of current is found at the anode, however. This surprising resist

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*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.


tive 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

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Mercury Vapor Lamp in recent issue of The
TECHNICAL World, above referred to.)

From the fact that an initial extra volt-
age is required to produce the state of low
negative electrode resistance, a vapor
verter can be constructed which will change
alternating current into direct current.

3. Owing to this same property, the ap-
paratus 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.

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Direct from Alternating Current
To make clear the usefulness of the
vapor converter, it will be necessary to
state briefly the characteristics of alter-

nating as distinguished from direct cur-


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 diestablish itself. The reason that older rectic: throughout the whole circuit, and types of vacuum apparatus do not act in at a later instant of time in the opposite the same manner as Mr. Hewitt's tube, is

direction throughout the whole circuit ; that the supply of current is intermittent and so on. Only direct current is suitin its nature, and that the negative elec

able for charging storage batteries. Ditrode resistance is being continuously re rect current is much more suitable than established by the interruption of the alternating for operating a great many current. Further, it is found that a liquid ciasses of motors, such as elevator and negative, such as mercury, is the only hoisting motors, and, up to the present one that will operate continuously with

time, motors for street-car or railway out deterioration. The old types of ap work. It is found, however, that alterparatus practically all use solid negatives. nating current is very much more eco

Mr. Hewitt discovered that current vomical than direct current for transpassing through a vacuum charged with mitting and distributing electrical power. mercury vapor causes this vapor to emit Consequently, since only direct current an intense light, with a comparatively is suitable for many purposes, it will small expenditure of energy.

Ilis dis often be advantageous to be able to obcovery of how the negative electrode tur direct current from alternating. resistance can be eliminated, allowed him This result can be accomplished, at the to make economical use of this very effi present time, by means of certain eleccient and new method of producing trical apparatus called the “rotary conlight. Hence his mercury vapor lamp. verter,” which is very similar to an elecApplications

tric generator or motor, but which is The great principle of the “negative handling, and is not particularly efficient.

large and expensive, requires expert electrode resistance” has thus been briefly explained. Various practical ap

It is the function of the vapor converter plications can be made of it, which may

to provide a more satisfactory method of be summarized as follows:

obtaining direct current when only an

alternating-current service is available. 1. Owing to the low operating "negative electrode resistance," a vacuum electric lamp

In the diagram a. Fig. 3, are shown can be economically used. (See description of

the variations of ordinary alternating




current or alternating electromotive 3, at point A, where we shall assume force. Consider that in this figure the the alternating current begins, it will be distance of the curved line above or be observed that the value of current begins low the horizontal line indicates the to decrease, until finally a zero point is strength of the current or voltage at any

reached at B. At this time, of course, particular instant of time, and that, the current ceases to flow. Meanwhile the further we go to the right on the hori- alternating electromotive force or voltzontal line, the later the instant of time age is reversed, and begins to increase in indicated. When the curve is above the strength. This reversal of direction horizontal line, the current in the cir makes the other electrode of the appacuit, or the electromotive force, is in one ratus the negative; and, if we provide direction; and when the curve is below no means of overcoming its initial negathe horizontal line, it is in the other di tive electrode resistance, no current can rection. It will be noticed that in chang flow. The same condition will exist also ing from one direction to the other, the when the alternating force has again value of the current or voltage gradually become zero, and flows in its original decreases to zero, and then increases in direction, as indicated at the point C the other direction up to a maximum, (Fig. 3). If the electrode resistance when it again decreases to zero, and re were broken down again at this point C, turns to its original direction; and so on. of course, the lamp would start again,

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.


AND POLYPHASE ELECTROMOTIVE FORCES. Such a converter, with four positives and a negative, is shown in Fig. 8.

On account of the nature of the nega and would run until the current again tive electrode resistance, if we connect a became zero at the point D. It is thus Hewitt mercury vapor apparatus across evident that this apparatus cannot run suitable direct-current mains, and start it continuously of itself on alternating curby breaking down the negative electrode rent. resistance, current will flow steadily Suppose, however, instead of the oruntil it is stopped by means external to dinary lamp, which has but two electhe apparatus, this being the proper op trodes, we use an apparatus like that eration of the apparatus on direct cur shown in Fig. 2, which has two electrodes rent for giving light.

at one end, and one at the other. SupSuppose, however, we suddenly change pose we pass direct current between electhe direct current to alternating current. trodes and 3 in such a direction that 3 At the instant of change, nothing new oc is the negative or cathode. Then, as in curs. There is no difference in the op the mercury vapor lamp, current will eration of the apparatus, since we shall continue to flow between these elecassume there is at the instant no change trodes; and the negative electrode rein the current. Then, referring to Fig. sistance of electrode 3 will be practically

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FIG. 3.

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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

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 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

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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 attempts 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 figure it is assumed that there is no choke the current away from F, and carries it coil to smooth out the variations of the to the point P, at which the electromotive current; whenever such choke coils are force E, having gone through a com used, the current is materially steadied. plete half-cycle, is ready again to take up On many alternating-current circuits, the current from G and carry it to the however, only one voltage is available, point Q; and so on indefinitely. With and there is no direct current for bridgthis type of circuit there is evidently no ing the zero points as was found necestime at which the current through the sary in Fig. 2. For these cases, it is very negative electrode becomes zero; and, desirable to have some means of operatconsequently, the converter continues to ing a vapor converter. This can be acoperate indefinitely without the neces complished by the apparatus shown in sity of supplying direct current.

Fig. 4, which is the same as that of Fig. Such three-phase electromotive forces 2, except for the connection to the supas are here described can be obtained ply circuit, and except for the choke coil from any three-phase circuit in which placed

placed in lead from the negative electhe neutral point is available. This is trode. the type of converter first shown by Mr. Referring to the figure, suppose that Hewitt. It will readily be seen that in the voltage between the electrode P and this case the direct current in the nega the middle point R of the supply transtive electrode—which is the current that former be supplying current through is available for useful work-has a much electrode 1; this current is delayed besteadier value than would be obtained hind the electromotive force, since it has by the apparatus of Fig. 2. Similarly, to flow through the choke coil. This with an apparatus having still another means that when the electromotive force positive electrode and four alternating from 1 has dropped to zero, the current current electromotive forces, we should has not yet reached zero, since it lags again have a converter in which the elec 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 i, and, on account of the lag of the current from 1, it will pick up this current before it becomes zero, and maintain it through the 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 be

fore it actually becomes zero, preventing tromotive forces so overlapped as to re the converter from stopping; and so on. quire no direct current for bridging the The simple introduction of the choke zero points of the current. Such alter coil enables the apparatus of Fig. 2, when nating electromotive forces can be ob- supplied by the proper electromotive tained from any two-phase or 14-phase force from a single-phase circuit, to opcircuit where the neutral point is avail erate indefinitely without the use of auxable. In this apparatus the current in iliary direct current. This is one of the the negative is still more nearly steady most important applications of the conthan in the case of the three-phase con verter. On account of the choking power verter.

of the coil, this arrangement gives quite The currents from the different types a steady current; but, as in most of the of alternating-current circuit are com cases above, it requires the neutral point pared as to steadiness in Fig. 5. In this of the alternating-current supply.


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