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The vapor converter itself has a num converter can also be arranged to be ber of interesting characteristics. Its started automatically. vacuum is similar to that of the mercury These converters will be especially usevapor lamp ; on the other hand, since it ful for charging storage batteries from is not intended to give light, its length is alternating-current supply circuits; for as short as possible ; this means that the running small direct-current motors; and, voltage across the converter, which rep- when built in larger sizes, for operating resents its total resistance, is reduced to railway motors from alternating-current 10 or 15 volts. As in the mercury vapor circuits. lamp, this voltage is practically constant,

As a Circuit-Breaker regardless of current, except for small The third application of the Hewitt current. On the other hand, since with mercury vapor apparatus is its use as a large currents in the container there is a Circuit-Breaker for alternating currents. considerable loss of energy, there is de- It is of course necessary, in all electric veloped within the enclosing chamber a plants, to have means for connecting and considerable amount of heat, which must disconnecting motors, lights, circuits, etc., be dissipated. It is therefore necessary as may be required; and it is usually very to make the container itself of consider 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 apparatus.

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 Fig. 6. MERCURY VAPOR Circuit-BREAKER.

through the vapor apparatus, going in by 6- Circuit Open,

one lead, passing through the mercury

from one cup to the other, and passing able size to get cooling surface; the mer out on the other lead. We have thus an cury of the negative electrode is evap- ordinary metallic circuit through the aporated during the operation, and con paratus; and, as long as the circuitdenses on the surface, flowing back again breaker remains in this position, current to the electrode.

continues to pass as though it were not a It is necessary to provide some means vacuum apparatus. for the starting of the converter. This is Next, suppose it is desired to stop the accomplished in a number of ways, in- flow of current; the circuit-breaker is volving the same principles as the start then tipped into the position shown in b, ing of the mercury vapor lamp. The Fig. 6, in which the mercury separates

a --- Circuit Closed.

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. cuit-breakers, for all contacts are made As was explained in the description of by mercury which, being in a vacuum, the mercury vapor lamp, when a sepa- cannot become corroded; consequently ration is made between the negative and there are no bad contacts, and no solid the positive electrodes which have been switch-tips to become burred or burned. in contact and are carrying a current, the The whole apparatus is so small as to be negative electrode resistance does not readily operated automatically. form, and the current passes freely In the mercury vapor circuit-breaker, through the vapor. The current breaker some interesting problems in construction is thus for the moment operated as a mer- arise. For example: It is difficult to get cury vapor lamp, and will continue thus the current, when of considerable volto operate until the current becomes zero. ume, from the external circuit, through Since we are dealing with an alternating the glass sealing chamber, to the mercurrent, the current will become zero cury within, the difficulty being the tenat the end of the first alternation; and dency of the current to heat the leadingwhen, in its natural course, the voltage in wire, causing it to expand and crack rises in the opposite direction, the nega- the glass. It is possible, however, withtive electrode resistance has become re- out serious mechanical complications, to established, and current cannot flow. As introduce a very large current into the a result, alternating current has been in- vacuum without causing any leaking terrupted. As long as the circuit-breaker 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

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.

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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

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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

By WALDON FAWCETT

REAT BRITAIN, eager to main- be profitable to Americans, for, aside

tain the prestige accruing from from the ties which bind the two naher position as the leading naval tions, there is the fact that a considerable

nation of the globe, is spending portion of the foreign seamen under the 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

SEAMEN AT SINGLE-Stick EXERCISE.

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THE HOPE OF THE BRITISH NAVY, Young seamen at physical drill, Portsmouth Dockyard. The ships here shown are a small portion of the Fleet Reserve.

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