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than the frequency of 25 cycles. Where the latter frequency has been retained it has been found preferable to use mechanical reduction gearing up to capacities of about 5,000 kw. Reduction gear may indeed be said to have revolutionized turbine driving for small outputs, the loss in the gearing being more than compensated by the increased efficiency of the high-speed steam turbine. It has further to be remarked that the application of reduction gearing to electrical work is still in its infancy. The greater expense of the geared drive is considered by many to be justifiable on account of its greater reliability and the higher efficiency of the plant.

The development of the continuous-current turbo-generator could not keep pace with the demand for increased output. Though satisfactory units up to 1,000 kw. were built, continuous-current turbo-generators are seldom built at the present day, except for installation on board ship. The demands of large users of continuous-current power such as railways, chemical works, etc., are best met either bv geared generators (steam turbines driving continuous-current generators through double helical reduction gearing) for moderate outputs, or by rotary converters for large outputs. Units of 2,000 to 15,000 kw. are not uncommon.

Both machines and transformers owe much of their development to the further utilization of the means for reducing the losses which occur in the iron and the copper. The use of silicon and other elements in alloy with steel in order to increase the resistance to the flow of eddy currents in iron is the factor which has been mainly responsible for the reduced weight per kva. of transformers, whilst the devices adopted for diminishing the unequal distribution of cur- rent in machines and transformers have rendered possible many modern designs.

As an instance of a modern power station may be cited that at Zschornewitz (Golpa), which at the present time (1921) is the largest steam-driven station in the world. This was erected in 1915 during the course of the war at the instance of the German Government for the supply of power for the production of nitrate of calcium in order to ensure a sufficient home supply of nitrates for agriculture and other necessary purposes. The engine-room contains 8 steam- turbine sets, each of 22,000 kva. capacity at 1,500 revolutions per minute, and the magnitude of the output may be judged from the daily consumption of about 7,000 tons of coal obtained from the lignite coal-field in the area of which the station is situated. There are 64 very large tubular boilers with 9 chimneys, each 328 ft. high, and II large cooling towers. Current is generated at 6,600 volts; of the total output 6,400 kw. are supplied at 6,000 volts to the nitrate works, while 33,000 kw. are supplied to Berlin, 95 m. distant, through a 100,000-volt double transmission line to a receiving station at Rummelsburg. The State is erecting at Friedrichsfelde a large distributing station for Berlin and adjoining districts, and at this station the combined outputs of the power stations at Lauta (40,000 kw.) and Spremberg (20,000 kw.) and from the Golpa transmission will be dealt with, while a third generating station in the Lausitzer lignite coal-field is in contemplation.

The lay-out of the plant in modern stations has been mainly governed by principles of economy. Larger boilers, higher steam pressures, greater superheat, the substitution of a small number of large turbine-driven sets for a large number of small slow-speed sets have all helped in this direction. The design and arrangement of the switch-gear have also been matters on which much care has been bestowed, particularly in countries where high transmission pressures up to 100,000 or even 150,000 volts have been adopted. In this connexion more efficient protection against lightning, pressure surges, short circuits, faults to earth, etc., may be particularly mentioned. The transformer is now built for such large powers and high pressures that, as with the switch-gear, separate housing is essential.

The cooling of the machinery and transformers calls for special consideration in the lay-out of large plants. Air is still the common cooling medium for machines, but the quantities needed by modern turbo-generators are so large that special intakes and outlets have to be provided. In addition, measures have to be taken for cleaning the air, particularly near towns or industrial centres. For this purpose dry filters were first tried, but were rapidly replaced by wet filters; that a completely satisfactory solution has not been attained thereby is evident from the experiments now being made to circulate the same air through the machine and a refrigerator. With transformers the case is somewhat different; oil is here the cooling medium, and air-blast transformers are now seldom called for. With natural oil-cooling no special provision has to be made, but in larger transformers usually the oil is water-cooled either by passing water through a cooling coil immersed in the upper part of the oil or by pumping the oil through a cooling chamber.

When continuous current is required it is often customary to generate 3-phase alternating current at the pressure required at the slip rings of the rotary converters, thereby dispensing with transformers. An important feature in connexion with modern switch-gear is the mistake-proof devices for preventing wrong connexions or danger to the operators.

The valid reasons upon which the electrification of railways may be advocated have now become more clearly defined, and progress has been made as these reasons have shown themselves to be applicable to specific cases. Before the World War there was a pronounced desire in certain countries to make themselves economically independent, and therefore to utilize available water-power rather than to import coal, although it was not always easy to show that any appreciable saving would accrue from electrifying railways under these conditions. The countries chiefly concerned in this way were Italy, Switzerland and Sweden. A great impetus, however, was given to this movement during the war on account of the scarcity and high price of coal, and a stage has now been reached when it is safe to say that whatever the cost of coal may be in the future, certain railway lines will no longer be worked by imported coal. Another great factor has been the difficulty of dealing with increased traffic. The introduction of the electric locomotive—by increasing the average speed, especially on inclines, and by rendering heavier train loads feasible—has in several cases proved a cheaper solution than doubling or quadrupling the track. The tunnel and terminal advantages will also be recognized.

As an indication of the importance that the electrification of main lines has assumed, reference may be made to the fact that in many countries the question has been taken up by the states concerned. The outstanding feature of all the reports and discussions that have appeared has been the debatable question of the best system. As far as can be seen at present, different countries will ultimately decide in favour of different systems.

The three systems which call for discussion are:—

(a) The three-phase system;

(b) the single-phase system;

(c) the continuous-current system.

From a technical standpoint, all three systems may be said to be satisfactory. It will now be convenient to deal with the several countries separately.

Great Britain.—The general electrification of railways has been discussed, but has hardly received serious consideration. In 1920, a committee was appointed to advise the Ministry of Transport, and in its interim report advocated as the standard system the continuous-current system at 1,500 volts, the mode of generation of the power to be that prevailing in the district. Up to the present, practically the only lines that have been electrified have been city and suburban railways in and around London, Liverpool, Newcastle- upon-Tyne and Manchester. Until recently, the 6oo-volt continuous-current system, as used on tramways, was adopted for the railways, but with a third rail instead of an overhead conductor. There are now two exceptions—the Newport-Shilton mineral line 18 m. long at 1,500 volts with an overhead conductor, and the Manchester-Bury line 10 m. long with 1,200 volts and a third rail. There are only two examples of the single-phase system—the important electrification of the suburban system of the London, Brighton and South Coast railway, with an overhead conductor at 7,000 volts and a frequency of 25 cycles per second, and the small Morecambe-Heysham experimental line on the Midland railway. Extensions on the Brighton system were in progress before the World War, but these were not completed in 1921. With the exception of a few electric locomotives for hauling passenger coaches and goods trucks over the electrified sections, motor coaches are used entirely on the English electric railways. Amongst recent extensions of the 6oo-volt system in and around London may be mentioned the electrification of the suburban lines of the London and South-Western railway, the extension of the London and North-Western railway electrification to Watford, and the extension of the Central London railway on the Great Western railway from Shepherd's Bush to Baling.

United States of America.—In the United States where so much has been done to develop both the continuous-current and the single-phase systems, many important electrifications have been carried out on both systems; but of late years, the leading firms, the General Electric Co. and the Westinghouse Co., appear to have favoured the continuous-current system. In America a break away from 600 volts was made long ago, and electrifications with 1,200 and 1,500 volts became quite common. Of recent years, the Butte-Anaconda mineral line was equipped on the continuous-current system at 2,400 volts and served as an experiment for the electrification of the Chicago, Milwaukee and St. Paul railway at 3,000 volts. This line, over 655 m., was in 1921 the longest in existence, but conditions on this mountainous line through the Rockies differ considerably from conditions in densely populated areas. With the possibility of one train in about every two hours, it is hard to draw comparisons with the New York Central, the Pennsylvania and the New York, New Haven and Hartford lines.

The single-phase system has also been extensively applied in the United States, particularly on the Philadelphia section of the