OcrToBER 9, 1913] 
NATURE 
189 
a natural reluctance against the use of high-pressure | 
on a commutator, designers are giving increased atten- 
tion to the use of continuous current for electric trac- 
tion. The difficulties which some engineers anticipate 
with commutator and brushes seem, however, rather 
imaginary than real, if we may judge from the experi- 
ence with the 3500-volt motor coach. The makers 
inform me that they estimate the mileage for a set 
of carbon brushes at 50,000 miles. The motors drive 
the car-axles by single reduction gear, and are con- 
trolled by contactors operated from a master con- 
troller. 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 3500 volts, 
and delivering C.C. at 210 volts. All motors have 
commutating poles—a practice which has become 
universal in C.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 2000 
to 3000 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 whatever 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 oscillograph, 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 improve the telephonic service further 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 along- 
side 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 dangev, first applied on some 
Swedish experimental lines, is to short-circuit thé 
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 regula- 
tion. There are no contactors and no rheostats. used, 
the power and speed of the motors being adjusted by 
the use of tapping on the secondary side of the trans- 
formers. 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 in- 
crease the weight, whilst at the same time it effects 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 
sometimes very complicated, arrangements for over- 
coming 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. 
NO. 2293, VOL. 92| 
Broadly speaking, only two methods are in use; the one 
is the method first made known by Messrs. Winter and 
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 corapensating or commutating 
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 
jepends 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 7 volts between heel and toe of brush, 
and a current density of 15 A. per sq. cm. is permissible. 
If, then, we use narrow brushes, covering ai 
any time not more than three segments, use coils of 
enly one turn to each segment, and work at a reason- 
ably low frequency, and not too high a total flux, it is 
possible to keep the transformer voltage and current 
density well within the above limits. This is not a 
severe limitation, for it enables the designer to use a 
flux out of one pole of 2°4 megalines if the frequency 
is 25, and 36 megalines if it is 15. The number of 
poles has then to be selected in accordance with the 
power desired. Obviously the lower periodicity is to 
be preferred, because the motor may be built with a 
lesser number of poles, and will then occupy less 
room—a matter of considerable importance consider- 
ing the limited space which is available in an electro- 
motive. The frequency of 15 has also some other 
advantages over that of 25. The e.m.f. of self-induc- 
tion is proportionately less, and, in consequence, the 
power-factor is about 5 per cent. better. The skin 
effect in the rails is much reduced, and also disturb- 
ances on neighbouring circuits which may be due to 
inductive or capacity effects. On the other hand, the 
generators become a little more expensive and the 
transformers on the electromotives a little heavier. 
But, notwithstanding these drawbacks, the balance 
of advantage is with the lower frequency, and that is 
the reason why the Commission of Experts called 
together in 1904 by the Swiss Government to establish 
standards for the electrification of the Swiss railways 
has decided that 15 shall be the standard frequency, 
with a tolerance down to 14, and up to 163. Since 
then other States have fallen into line, so that 15 is 
now the standard frequency nearly all over the con- 
tinent of Europe. The standard pressure is likely to 
be 15,000 volts. For three-phase tractions the standard 
pressure is 3000 to 3300 volts. 
The subject of electric main-line traction is so vast 
that in the limited time at my disposal I have only 
been able to mention a few of the important features 
of this interesting problem. A detailed account of 
all that has been done in electrification would take far 
more time than we can spare; but, by way of example, 
I give below two tables referring to the Italian State 
Railways. I am indebted for the information to Mr. 
v. Kando, who may justly be described as the father 
of three-phase traction. 
