186 
NATURE 
[OcTOBER 9, 1913 
place of the zero wire. Each electromotive is fitted 
with four motors, of which at least two are in series, 
taking 1500 volts. Thus, whilst the voltage of one 
motor is kept within the customary limit of 750 volts, 
the pressure of the whole system is 3000 volts. The 
objection to this arrangement 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 intro- 
duce the very feature which the advocates of the 
continuous-current system find so objectionable in 
three-phase traction. Moreover, the recent improve- 
ments made in continuous-current motors has reduced 
the importante of the three-wire principle. | Con- 
tinental makers are prepared to build motors for 1200 
volts, and one English maker is actually building 
motors for 1750 volts, so that with two motors in 
series a trolley-pressure of 2400 and 3500 volts respec- 
tively 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 isavoided. Only three tvpes of current 
are used—namely continuous, 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 com- 
pleted 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 develop- 
ment—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 opera- 
tion, and the Rhztian Alp Railway, nearing com- 
pletion, 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 rail- 
way company ordered a 2000 horse-power electro- 
motive, 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 2500 horse- 
power on the 13-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 Govern- 
ment took over the railways, the Government 
engineers decided to extend the application of three- 
NO. 2293, VOL. 92] 
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-power (95,000 horse-power 
in service, and 120,000 horse-power building). Ten 
years ago the three-phase system was the only pos- 
sible one for main-line working, but later on there 
came on the scene the single-phase, and, later still, 
the high-pressure continuous-current systems, and I 
need scarcely mention that between the advocates of 
the three systems there has been waged a fierce battle, 
each claiming that his is the best and the others very 
inferior. I am afraid that battle is still raging; but it 
is a futile war, for there is no such thing as a best 
system generally. One system is the best for one 
set of conditions and another for another set. Thus 
the German railway engineers found that the single- 
phase system would serve them best, and they adopted 
it. There is in this matter no question of personal 
feeling or national prejudice. I have no intention 
to enter the lists as an advocate for any one of the 
three possible systems for main-line traction; each has 
its special features and special merits, and all I can 
do is to place before you some of these. As the three- 
phase system is the oldest, it will be convenient to 
take it first. 
It is curious to note that the three most obvious 
objections which have been raised against three-phase 
electromotives by theorists have been found to have 
but little weight in practical work. These objections 
were: the complication of a double overhead wire, 
the danger that the motors would not share the load 
fairly, and the inability to run without rheostatic waste 
at intermediate speeds, or to run at a higher than 
synchronous speed to make up for lost time. 
That an overhead wire is inconvenient must be 
readily admitted, but the inconvenience applies to all 
methods of main-line working, for the so-called third 
rail is not applicable to high pressure, and even if it 
were, the consideration of the safety of the platelayer 
would preclude its use. The question then is: are 
two wires twice as objectionable as one? Possibly, 
but the most objectionable feature is not the wire 
itself, but the posts or gantries on which it is carried, 
and the number of posts is the same, whether we 
use three-phase, single-phase, or continuous current. 
There is a little more complication at the cross-over 
points and at the switches; but this is not a serious 
matter, if one may judge from the perfectly smooth 
working of so extended a yard as that at Busalla, 
where there are five miles of track, connected by 
thirty-seven switches and crossings. The other objec- 
tion—as to the motors not sharing the load equally 
—-is theoretically sound. The torque developed by the 
motor is proportional to the slip, and in order that the 
two motors on an electromotive shall share the load 
equally their slips, and consequently also their speeds, 
must be the same. Now, it is conceivable that, owing 
to a slight difference in the size of the drivers, that 
motor which is geared to the larger drivers will, by 
reason of its lower speed and consequently greater 
slip, take more than its fair share of the load. In 
practice this difficulty does, however, not arise. With 
reasonably good workmanship there should be no 
sensible difference in the size of the wheels; but even 
if we admit the possibility of there being a difference 
of 4 per cent. in the diameter of the wheels, this 
would, with the usual slip of 3 per cent., only mean 
that the motor geared to the larger wheels develops 
8 per cent. more, and the other 8 per cent. less, than 
its normal power. The larger wheels will develop 
16 per cent. more tractive effort than the smaller 
wheels, and having thus a greater wear, the differ- 
