590 TRANSACTIONS OF SECTION G. 
been so much improved that the margin of weight that could be saved by the use 
of continuous-current motors is probably less than the excess weight of the 
converting machine. 
The other attempt to combine high trolley-voltage with low motor-voltage 
has shared the same fate. This consisted in the application of the three-wire 
principle of continuous current supply to electric traction. It is in successful 
operation at a moderate voltage on a London tube railway, but as far as main- 
line working is concerned it has not got beyond an application on two small 
lines in Bohemia. The principle adopted is to make the trolley wire of the up- 
line the positive and that of the down-line the negative side of the system, 
whilst the rails take the place of the zero wire. Each electromotive is fitted 
with four motors, of which at least two are in series, taking 1,500 volts. Thus, 
whilst the voltage of one motor is kept within the customary limit of 750 volts, 
the pressure of the whole system is 3,000 volts. The objection to this arrange- 
ment is that its fundamental supposition of a fairly close balance between the 
two halves of the three-wire system must in actual railway working be rather 
the exception than the rule, and that the obvious remedy of combining both 
halves of the system in one and the same train would involve the use of two 
overhead trolley wires, and thus introduce the very feature which the advocates 
of the continuous-current system find so objectionable in three-phase traction. 
Moreover, the recent improvements made in continuous-current motors has 
reduced the importance of the three-wire principle. Continental makers are 
prepared to build motors for 1,200 volts, and one English maker is actually 
building motors for 1,750 volts, so that with two motors in series a trolley- 
pressure of 2,400 and 3,500 volts respectively can be used. 
The present tendency in electric traction is in the direction of simplicity, in 
the sense that mixing up of different types of current and dependence of one 
train on another is avoided. Only three types of current are used—namely, con- 
tinuous, three-phase, and single-phase. The two first-named are used direct; 
the last through the intervention of a transformer. In a large measure the 
different systems have already become standardised. As regards the C.C. 
system, up to 750 volts the process of standardisation has been completed long 
ago. It is almost generally adopted for urban and suburban lines of moderate 
length, unless there are local difficulties as regards the third rail, or it is desired 
to work the suburban and the main-line service on the same system. The three- 
phase system has also been fairly well standardised, but the single-phase system 
is still in a process of development—a development which, however, takes place 
on a fairly large scale. In France the Compagnie du Midi is electrifying on the 
single-phase system nearly 400 miles of track; the German Government have 
already electrified the Dessau-Bitterfeld of the Leipzig-Magdeburg line, and are 
electrifying the line Lauban-Koenigszelt in Silesia, to say nothing of some 
smaller private lines in the South of Germany, which have been in operation for 
some years. In Switzerland the Berne-Loetschberg-Simplon Railway, already in 
operation, and the Rhetian Alp Railway, nearing completion, also employ single- 
phase electromotives. Both in France and Germany the type of electromotive to 
be finally adopted has not yet been settled, but half-a-dozen different types, 
supplied by as many different makers, are being tried, and it is in this respect 
that one may look on single-phase traction as still in the process of development. 
As regards the Loetschberg the period of trial is over. Three years ago the 
railway company ordered a 2,000 horse-power electromotive, and have had it at 
work ever since with such satisfactory results that they have decided to adopt 
this type definitely, and have ordered thirteen more engines, but of the slightly 
larger power of 2,500 horse-power on the 14-hour rating. Of these I shall have 
to say something more presently ; but before entering into the details of single- 
phase traction it is expedient to glance briefly at the present position of the 
rival system of three-phase traction. 
The first application of this method of working dates back to the end of last 
century, and took place on a small Swiss line; then followed the well-known 
Valtellin line, and, later still, when the Italian Government took over the rail- 
ways, the Government engineers decided to extend the application of three-phase 
traction to some other lines—a decision which practical experience has shown to 
have been perfectly justified. The total power represented by three-phase 
electromotives either at work or on ‘order in Italy to-day exceeds 200,000 horse- 
