G.—ENGINEERING. 185 
The development of these robust and weather- proof switches and trans- 
formers has led to the introduction of the open-air sub-station in cases 
where alternating current has to be transformed from one voltage to the 
other, and there is consequently no running machinery. In generating 
stations also much of the controlling and transforming plant which was 
formerly housed in the building can now be placed outside, with consider- 
able saving on the cost of the building. 
In connection with the conversion of alternating to direct current, 
mention should be made of the mercury arc rectifier. Great improve- 
ments have been made in recent years, especially in Switzerland, and a 
number of high-power arcs have been installed in sub-stations. Although 
they have the advantage of doing away with running machinery, the 
modern rotary-converter is such a reliable piece of apparatus that it is 
very questionable whether it will be replaced to any considerable extent 
by the mercury arc rectifier. 
Until recently, the only means of producing a large amount of high- 
voltage D.C. power was by connecting a large number of carefully insulated 
dynamos in series, as in the well-known Thury system of power trans- 
mission. Within the last two or three years another method has been 
developed, viz., the so-called transverter, which consists of an arrange- 
ment of transformers and a system of rotating brushes, whereby a three- 
phase A.C. supply is converted into an almost steady continuous current. 
The first apparatus of this type to be exhibited is installed at the British 
Empire Exhibition at Wembley, and is designed to deliver continuous 
current at 100,000 volts. It can also be used for the reverse process. It 
would thus enable a three-phase generating station and a three-phase sub- 
station to be connected by a direct-current transmission line, thus avoiding 
not only the maximum voltage of 1-4 times the effective voltage, which 
was one of Lord Kelvin’s objections to the A.C. system, but also all 
trouble due to the capacity and inductance of the line. Whether the dis- 
advantages of the transverter, when it is fully developed—it is yet in its 
infancy—will more than outweigh these advantages remains to be seen, 
but, apart from the transmission of power, the device may have many 
applications, 
Electric traction represents one of the most important branches of 
electrical engineering. It shares with the petrol motor the distinction of 
having absolutely revolutionised the methods of transport within a single 
generation. In its origins it is nearly a century old, for attempts were 
made in the thirties to apply Faraday’s newly discovered principle to the 
propulsion of vehicles, but, with very primitive motors and primary 
batteries, these attempts were doomed to failure. The development of 
the dynamo and motor in the seventies opened the way to further 
experiments, and at the Berlin Exhibition in 1879 a line one-third of a 
mile long was shown in operation, a locomotive drawing three cars. The 
first regulai line was opened to traffic near Berlin in 1881 ; it worked at 
100 volts and the current was collected from an insulated rail. Toronto was 
the scene of one of the earliest experiments in America; C. J. van Depoele, 
after some experiments at Chicago in 1882 and 1883, ran an electric 
locomotive in 1884 between the street-car system and the Exhibition in 
Toronto. 
The difficulties were enormous. The carbon brush was not invented 
