Page images
PDF
EPUB

The vapor converter itself has a number of interesting characteristics. Its vacuum is similar to that of the mercury vapor lamp; on the other hand, since it is not intended to give light, its length is as short as possible; this means that the voltage across the converter, which represents its total resistance, is reduced to 10 or 15 volts. As in the mercury vapor lamp, this voltage is practically constant, regardless of current, except for small current. On the other hand, since with large currents in the container there is a considerable loss of energy, there is developed within the enclosing chamber a considerable amount of heat, which must be dissipated. It is therefore necessary to make the container itself of consider

[blocks in formation]

converter can also be arranged to be started automatically.

These converters will be especially useful for charging storage batteries from alternating-current supply circuits; for running small direct-current motors; and, when built in larger sizes, for operating railway motors from alternating-current circuits.

As a Circuit-Breaker

The third application of the Hewitt mercury vapor apparatus is its use as a Circuit-Breaker for alternating currents.

It is of course necessary, in all electric plants, to have means for connecting and disconnecting motors, lights, circuits, etc., as may be required; and it is usually very desirable that means should be provided for automatically stopping the flow of current in any circuit when any accident occurs to the apparatus on this circuit. These functions are performed by many different types of switches and circuitbreakers, which, on large high-tension plants, are very cumbersome and expensive, especially when automatic. In many plants, the size of the switching apparatus is so excessive that it is impossible to place the switches and circuit-breakers for operating the plant within view of the operator, who must rely on indicating devices to determine the condition of his circuits. On the principle of the mercury vapor apparatus, it is possible to construct a circuit-breaker which will accomplish this same work with a very much less cumbersome and bulky appa

ratus.

To understand how the principle of the negative electrode resistance may be applied to this apparatus, we may consider the circuit-breaker in a, Fig. 6. Suppose that the current, in flowing from the generator to the circuit, passes through the vapor apparatus, going in by one lead, passing through the mercury from one cup to the other, and passing out on the other lead. We have thus an ordinary metallic circuit through the apparatus; and, as long as the circuitbreaker remains in this position, current continues to pass as though it were not a vacuum apparatus.

Next, suppose it is desired to stop the flow of current; the circuit-breaker is then tipped into the position shown in b. Fig. 6, in which the mercury separates

between the two cups. At the moment of separation, the current, which has been flowing through the mercury from cup to cup, is obliged to pass through a portion of the vapor, making the mercury in one cup the negative electrode, and that in the other the positive, it being merely accidental which way the alternating current may happen to be flowing at the

remains in the position of b, Fig. 6, no current will flow. However, if it be desired to start the flow of current again, it is necessary merely to tilt back the circuit-breaker into the first position, when, since mercury now joins the two cups, current will flow unimpeded as at first. The mercury vapor device has other advantages over the present types of cir

FIG. 7. MERCURY VAPOR DISCHARGE GAP FOR HEAVY CURRENT.

instant of the separation of the mercury. As was explained in the description of the mercury vapor lamp, when a separation is made between the negative and the positive electrodes which have been in contact and are carrying a current, the negative electrode resistance does not form, and the current passes freely through the vapor. The current breaker is thus for the moment operated as a mercury vapor lamp, and will continue thus to operate until the current becomes zero. Since we are dealing with an alternating current, the current will become zero at the end of the first alternation; and when, in its natural course, the voltage rises in the opposite direction, the negative electrode resistance has become reestablished, and current cannot flow. As a result, alternating current has been interrupted. As long as the circuit-breaker

cuit-breakers, for all contacts are made by mercury which, being in a vacuum, cannot become corroded; consequently there are no bad contacts, and no solid switch-tips to become burred or burned. The whole apparatus is so small as to be readily operated automatically.

In the mercury vapor circuit-breaker, some interesting problems in construction arise. For example: It is difficult to get the current, when of considerable volume, from the external circuit, through the glass sealing chamber, to the mercury within, the difficulty being the tendency of the current to heat the leadingin wire, causing it to expand and crack the glass. It is possible, however, without serious mechanical complications, to introduce a very large current into the vacuum without causing any leaking

seals.

As a Discharge Gap

The fourth application of the mercury vapor apparatus, which has been frequently described in the technical press, is the Discharge Gap, sometimes called the interrupter. The function of the discharge gap is easily explained. For the production of X-rays, the sending of wireless telegraph messages, and many other purposes, it is necessary to have extremely sudden discharges of high-po

FIG. 8. CONVERTER WITH FOUR MERCURY POSITIVE ELECTRODES.

tential electricity. These are always obtained by introducing into the circuit from which the discharge is to be obtained, an air-gap or its equivalent, and by raising the voltage until this air-gap is unable to resist the strain, and breaks down. At this instant, there is a very sudden rush through the circuit of the electricity which has been previously accumulated on both sides of the air-gap; and this sudden discharge serves to send the wireless message, or excite the X-ray tube.

But a disadvantage to be found with air-gaps is, that, even after they break down, they introduce a considerable resistance into the circuit, which pre

vents this discharge from being perfectly free as is most desirable. A further disadvantage to be met with in the air-gap is that an undesirably long time is required for it, when once it has been heated by the discharge, to cool sufficiently to be able to operate properly a second time. In addition, whenever these gaps are used for large quantities of energy, or continuously, the metal electrodes become scarred and burnt, and require constant replacing.

The mercury vapor apparatus shown in Fig. 7 may serve the same purpose as the air-gap, for, if we apply a comparatively low voltage between the two electrodes, no current will flow, because of negative electrode resistance, unless we raise the voltage sufficiently to overcome this resistance. Then the discharge of electricity will be practically free, since, when once broken down, the negative electrode resistance is substantially eliminated. Thus, by the use of the mercury vapor apparatus, the resistance to the discharge, introduced by an air gap, is eliminated.

Furthermore, it is found by trial, that, even with a series of the most rapid discharges, the negative electrode resistance can always be re-established between them, which is by no means true of the air-gap. And again, evidently, the burning and burring of the electrodes of the air-gap are eliminated, as the mercury cannot corrode in a vacuum.

The action of the mercury vapor discharge gap has been carefully studied by the rotating mirror method, and has been found to be free in a remarkable degree from the weaknesses of the air-spark gap. Fig. 7 shows a discharge gap used by Mr. Hewitt in his laboratory on some of his high-tension work. These bulbs are about five inches in diameter.

In addition to the applications so far described, there are others of less immediate importance, which space will not permit to be described here. It must be remembered that the discovery of the great principle of the negative electrode resistance is quite recent, and that it is a radically new phenomenon, and consequently capable of producing most unusual results.

[graphic]
[graphic]

Training of the British Tar

Foundation of Britannia's Maritime Supremacy-The Drilling of Seamen, Upon Whose Efficiency Chiefly Depends the Strength of the Empire's First Line of Defense

G

By WALDON FAWCETT

REAT BRITAIN, eager to maintain the prestige accruing from her position as the leading naval nation of the globe, is spending annually an immense sum of money in training young men for service in her Navy. Whether the system employed is calculated to give to the naval sailors of the United Kingdom as perfect an intellectual and physical equipment for their sea duties as is afforded by the methods of some other nations-notably the United States, is a question open to argument, and is to-day a topic of animated discussion in international naval circles. But whatever be its merits or demerits, the British system of training bluejackets is interesting and instructive. Particularly may observation in this field.

[merged small][graphic][merged small]
[graphic][merged small]

Young seamen at physical drill, Portsmouth Dockyard -The ships here shown are a small portion of the Fleet Reserve.

« PreviousContinue »