An Electric Iceberg Detector
Amongst the many dangers to which ships crossing the Atlantic are exposed is that of collision with icebergs. These are large masses of ice which have become detached from the mighty ice-fields of the north, and which travel slowly and majestically southwards, growing smaller and smaller as they pass into warmer seas. Icebergs give no warning of their coming, and in foggy weather, which is very prevalent in the regions where they are encountered, they are extremely difficult to see until they are at dangerously close quarters.
Attempts have been made to detect the proximity of icebergs by noting the variations in the temperature of the water. We naturally should expect the temperature of the water to become lower as we approach a large berg, and this is usually the case. On the other hand, it has been found that in many instances the temperature near an iceberg is quite as high as, and sometimes higher than the average temperature of the ocean. For this reason the temperature test, taken by itself, is not at all reliable. A much more certain test is that of the salinity or saltness of the water. Icebergs are formed from fresh water, and as they gradually melt during their southward journey the fresh water mixes with the sea water. Consequently the water around an iceberg is less salt than the water of the open ocean. The saltness of water may be determined by taking its specific gravity, or by various chemical processes; but while these tests are quite satisfactory when performed under laboratory conditions, they cannot be carried out at sea with any approach to accuracy. There is however an electrical test which can be applied accurately and continuously. The electrical conducting power of water varies greatly with the proportion of salt present. If the conductivity of normal Atlantic water be taken as 1000, then the conductivity of Thames water is 8, and that of distilled water about 1/22. The difference in conductivity between normal ocean water and water in the vicinity of an iceberg is therefore very great.
By permission of]
[Dr. Myer Coplans.
Fig. 43.—Diagram of Heat-compensated Salinometer.
The apparatus for detecting differences in salinity by measuring the conductivity of the water is called a “salinometer,” and its most perfect form, known as the heat-compensated conductivity salinometer, is due to Dr. Myer Coplans. [Fig. 43] shows a diagram of this interesting piece of apparatus, which is most ingeniously devised. Two insulated electrodes of copper, with platinum points, are suspended in a U-tube through which the sea water passes continuously, as indicated in the diagram. A steady current is passed through the column of water between the two platinum points, and the conductivity of this column is measured continuously by very accurate instruments. Variations in the conductivity, indicating corresponding variations in the saltness of the water, are thus shown immediately; but before these indications can be relied upon the instrument must be compensated for temperature, because the conductivity of the water increases with a rise, and decreases with a fall in temperature. This compensation is effected by the compound bars of brass and steel shown in the vessel at the right of the figure. These bars are connected with the wheel and disc from which the electrodes are suspended. When the temperature of the water rises, the bars contract, and exert a pull upon the wheel and disc, so that the electrodes are raised slightly in the U-tube. This increases the length of the column of water between the platinum points, and so increases the resistance, or, what amounts to the same thing, lowers the conductivity, in exact proportion to the rise in temperature. Similarly, a fall in temperature lowers the electrodes, and decreases the resistance by shortening the column of water. In this way the conductivity of the water remains constant so far as temperature is concerned, and it varies only with the saltness of the water. Under ordinary conditions a considerable decrease in the salinity of the water indicates the existence of ice in the near neighbourhood, but the geographical position of the ship has to be taken into account. Rivers such as the St. Lawrence pour vast quantities of fresh water into the ocean, and the resulting decrease in the saltness of the water within a considerable radius of the mouth of the river must be allowed for.
A “Flying Train”
Considerable interest was aroused last year by a model of a railway working upon a very remarkable system. This was the invention of Mr. Emile Bachelet, and the model was brought to London from the United States. The main principle upon which the system is based is interesting. About 1884, Professor Elihu Thompson, a famous American scientist, made the discovery that a plate of copper could be attracted or repelled by an electro-magnet. The effects took place at the moment when the magnetism was varied by suddenly switching the current on or off; the copper being repelled when the current was switched on, and attracted when it was switched off. Copper is a non-magnetic substance, and the attraction and repulsion are not ordinary magnetic effects, but are due to currents induced in the copper plate at the instant of producing or destroying the magnetism. The plate is attracted or repelled according to whether these induced currents flow in the same direction as, or in the opposite direction to, the current in the magnet coil. Brass and aluminium plates act in the same way as the copper plate, and the effects are produced equally well by exciting the magnet with alternating current, which, by changing its direction, changes the magnetism also. Of the two effects, the repulsion is much the stronger, especially if the variations in the magnetism take place very rapidly; and if a powerful and rapidly alternating current is used, the plate is repelled so strongly that it remains supported in mid-air above the magnet.