PRESIDENTIAL ADDRESS, 591 
power (95,000 eon ei in service, and 120,000 horse-power building). Ten 
years ago the three-phase system was the only possible 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 hardly 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 
cther objection—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 conceiv- 
able 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 con- 
sequently 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 a half per cent. in the diameter 
of the wheels, this would, with the usual slip of three per cent., only mean that 
the motor geared to the larger wheels develops eight per cent. more, and the other 
eight per cent. less, than its normal power. The larger wheels will develop 
sixteen per cent. more tractive effort than the smaller wheels, and having thus 
a greater wear, the difference originally existing will diminish in service. For 
the same reason, any tendency to wear unequally, say, in consequence of unequal 
material, is counteracted by the slip-adjustment of the motors. This point has 
been tested practically by the makers of the Simplon three-phase electromotives. 
It was found that if originally a slight difference in diameter of the drivers had 
been permitted to exist, after a short time this had vanished. That is as regards 
the condition on one electromotive; but if we come to the case of a train being 
hauled by two engines, then a sensible difference in the size of their wheels may 
exist. In this case it is necessary to artificially adjust the slip so as to make 
each motor take half the load. This problem has been solved by Mr. v. Kando 
in the electromotives which he designed for the Italian State railways. In these 
engines only liquid resistances are used in the rotor circuit for starting and speed 
regulation. The liquid is raised or lowered in the rheostat chambers so as to 
cover more or less of the contact plates, and the level of the liquid is controlled 
by a solenoid under the influence of the working current. ‘The working current, 
and therefore also the tractive effort exerted by each motor, is thereby auto- 
