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PRESIDENTIAL ADDRESS, 595 
The motors drive the car-axles by single reduction gear, and are controlled by 
contactors operated from a master controller. The current for operating the 
contactors, driving the air-pump motor, and for the general service of lighting 
and heating is obtained from a small motor-generator, fed on the primary side at 
3,500 volts, and delivering C.C. at 210 volts. All motors have commutating 
poles—a practice which has become universal in O.C. traction work. 
From the figures quoted above it will be seen that where motor coaches are 
employed the C.C. system has an advantage in point of weight over the single- 
phase A.C. system. But main-line traction, including goods trains, is not going 
to be done by motor coaches, and if we come to large electromotives of some 2,000 
to 3,000 horse-power then this advantage is likely to vanish. No high-pressure 
C.C. electromotive has as yet, been built for so large a power, and it is therefore 
not possible to make a direct comparison ; but, if we may judge from the largest 
engines yet built for moderate-pressure C.C., there is little probability that the 
C.C. system for high-pressure can beat the single-phase system, and none what- 
ever that it can beat the three-phase system. 
In the early days of single-phase traction some trouble has been experienced 
in the matter of telephonic disturbance. A systematic investigation carried on 
for over a year on the Seebach-Wettingen line, chiefly by means of the oscillo- 
graph, showed that this trouble was due, not as had originally been suspected, 
to the commutator, but to the employment of open slots in the rotor, and the 
trouble nearly ceased when new rotors with semi-closed and spiralled slots were 
used. To further improve the telephonic service the usual remedy of metallic 
return and drilling the telephone lines was employed. Although by these means 
it is possible to render telephonic speech over a line alongside a single-phase 
railway nearly, and perhaps quite, as clear as it is along a C.C. railway, there 
still remains the danger that the telephone lines may, by electrostatic induction, 
acquire a very high potential. The remedy against this danger, first applied 
on some Swedish experimental lines, is to short-circuit the two wires of each 
circuit by a choking coil of very high inductance, the centre of which is earthed. 
The static charge is thus carried off to earth, whilst the telephonic currents are 
only inappreciably weakened. 
One of the advantages possessed by the alternating over the continuous current 
is the simplicity of regulation. There are no contactors and no rheostats used, 
the power and speed of the motors being adjusted by the use of tappings on the 
secondary side of the transformers. As transformers are necessary in any case 
in order to work with a high voltage on the trolley, the introduction of tappings 
does not materially increase the weight, whilst at the same time it affects a great 
reduction in the primary starting current. The only difficulty that still remains 
is that of sparkless commutation, and inventors have evolved many, and some- 
times very complicated, arrangements for overcoming it. As so often happens 
with engineering problems, the most simple solution is, after all, found to be the 
best in practice; and of all the ingenious inventions patented during the last ten 
years very little use is made by the designer of traction motors. Broadly speak- 
ing, only two methods are in use; the one is the method first made known by 
Messrs. Winter & Eichberg, where the working field is produced by direct 
excitation of the rotor and the transformer e.m.f. in the coils short-circuited by 
the main brushes is balanced by an e.m.f. of rotation due to a transverse field ; 
and the other method applicable to the straightforward series motor, where a 
non-inductive shunt is connected to the terminals of the compensating or com- 
mutating winding. The effect of a non-inductive shunt is to make the armature 
field slightly leading over the field produced by the compensating winding. The 
resultant of these two fields is in position coincident with the brush axis, but 
has in point of time a phase difference of a quarter period over the working 
current, thus balancing the e.m.f. of self-induction, which lags by a quarter 
period. Obviously this balancing effect can only take place when the motor is 
running, since it depends on the balance between an e.m.f. of self-induction 
which is independent of speed and an e.m.f. of rotation which is proportional to 
speed. At starting, when there is no speed, there is no compensation. Thus 
there would appear to be a new difficulty in the way of the use of single-phase 
current; but also this has been overcome in quite a simple manner. Experience 
has shown that a potential difference of seven volts between heel and toe of brush, 
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