THE BIRTH OF ELECTRIC TRACTION
The story of electric traction really begins in the laboratory
of Faraday. He was the first to produce mechanical rotation by
electrical means; and, although he had no practical end in view, his
investigations produced the germ of the commercial dynamo and thence of
the commercial electric motor.
That germ, however, took about half a century to develop. It is true
that in 1837 (about ten years after Faraday's discovery) Robert
Davidson experimented with an electric locomotive on the Edinburgh
and Glasgow Railway; it is also true that Jacobi, two years later,
propelled a boat on the Neva with electric power. But these early
attempts were not on a commercial scale. Not only was the motor a
crude contrivance, but the method of producing the electric power was
hopelessly extravagant.
At that period the 'primary battery'--similar in character to those
still used for laboratory purposes, ringing electric bells, and so
on--was the best available source of electricity. Such batteries
generate current by the chemical consumption of zinc. In order to
obtain sufficient power to move a boat, a large number of batteries had
to be coupled together. They were expensive in first cost, expensive in
the zinc which was their 'fuel'; and they became rapidly exhausted.
[Illustration: =DYNAMO= =MOTOR=
Fig. 1. Diagram to illustrate the essential identity of the dynamo
and the motor. The dynamo generates electricity when the armature or
group of coils is forcibly revolved close to magnets, thus converting
mechanical energy into electrical energy. The motor causes its
armature to revolve forcibly when current is supplied to it from the
dynamo. Thus the motor converts electrical energy into mechanical
energy.]
The essential step towards the commercial plane was taken when an
efficient means was devised for transforming mechanical into electrical
energy on a large scale. The first 'dynamo-electric' machines, invented
about the middle of last century, were merely hand machines. Their
power was limited by the strength of the permanent magnets employed in
their construction; and although an increase in power was obtained by
multiplying the number of magnets and driving by steam power, it was
not sufficient for commercial purposes. In 1867 electro-magnets were
first employed by Siemens and Wheatstone; and from this application
there was developed a machine whose power as a generator of electricity
was limited only by its size and the speed at which it was run.
It is unnecessary for our present purpose to enter into the technical
details of the modern electric generator and the modern electric motor.
The principles underlying them are quite simple, although the theory
of their design and the practice of their construction and operation
are almost a science in themselves. A dynamo or electric generator is a
machine for transforming mechanical into electrical energy; an electric
motor is a machine for transforming electrical energy into mechanical
energy. If, therefore, we place an electric motor upon a vehicle and
supply it continuously with current from a dynamo, the motor will
rotate and can be used to propel the vehicle. That is the essential
mechanism of electric traction.
The simplicity of the arrangement is enhanced by the fact that the
dynamo and the motor are virtually the same machine. In the dynamo, a
cylindrical 'armature' of coils is forced to rotate close to the poles
of electro-magnets; the energy exerted in turning the armature against
the influence of the electro-magnets is transformed into the energy of
electric currents in the coils of the armature. In the motor, which
also consists of an armature close to the poles of electro-magnets, the
process is reversed. When a current is passed through the coils of the
armature, the reaction between these currents and the electro-magnets
causes the armature to revolve.
This reversibility of the dynamo was, according to a story frequently
repeated, first discovered quite by accident. In a Paris exhibition a
number of Gramme dynamos--or dynamo-electric machines, as they were
then called--were being separately connected to lamps and other devices
for showing the effect of electric currents; and when one was started
up it was found that another was being _driven_ at a rapid rate.
Investigation showed that the second one had been coupled up to the
first by mistake and was therefore being worked as a motor by it.
This was in the year 1879; and the story of the incident served to
draw general attention to the discovery of a new and efficient means
of transmitting power. Engineers recognised that in the steam-driven
dynamo they had the means of producing powerful electric currents,
while in the electric motor, connected by wires to the dynamo, they
had the means of reproducing the power in mechanical form at a
distance. There were, of course, losses of energy in the process.
A certain percentage was lost in the dynamo itself, some in the
transmitting wires, and some in the motor. But the all-round efficiency
of the arrangement was much higher than that of any other system of
transmitting power from one point to another several miles distant.
In order to apply this system to propelling vehicles it was only
necessary to devise a continuous connection between the motor on the
vehicle and the stationary dynamo. This was done on the first electric
railway by means of a 'third rail,' substantially in the same way as
is now familiar on underground and other electric lines. The third
rail was a metal conductor supported on insulators and connected to
the dynamo. The vehicle or car was furnished with a metal brush or
skate which rubbed along the third rail as the car moved forward. The
current thus collected was led through the motor (which drove the axle
of the car through toothed wheels) and thence to the track rails, which
conveyed the current back to the dynamo and so completed the electrical
circuit. Messrs Siemens and Halske exhibited the first electric railway
of this type at the Berlin Industrial Exhibition of 1879.
Another method of collecting the current was tried soon afterwards
and formed the direct forerunner of the electric tramway on the now
standard 'overhead' system. The disadvantage of the third rail system
is that it involves an exposed 'live' conductor close to the ground.
It is therefore quite unsuited for use on streets. Consequently the
next step towards the electric tramway was to carry the electrical
conductors overhead by supporting them on poles erected at the side of
the track. The first installation of this kind was laid down at the
Paris Exhibition of 1881. In that case the conductor was an iron tube
with a slot along its lower side; and inside the tube was a 'boat'
which slid along and was connected to the car by means of a flexible
wire. A second tube, also with a boat and connecting wire, was provided
to carry the return current. We shall see later how this arrangement
evolved into the familiar 'trolley' system.
The mention of a slotted tube recalls the atmospheric system and,
in so doing, emphasises the superiority of the electric system in
simplicity, flexibility, reliability, and economy. Brunel's faith in
the advantages of stationary engines and the transmission of power
therefrom to moving trains would have been justified by the event if
the pneumatic system of power transmission had been as practicable as
the electric system. But there is an obvious contrast between the huge
pipe of the atmospheric railway, with its impossible 'longitudinal
valve,' and the small tube of the first overhead electric line or the
third rail of the first electric railway. There is also a pathetic
contrast between the prolonged struggles which Brunel and the inventors
of the atmospheric system underwent before they were forced to
acknowledge failure, and the rapid ease with which electric traction
entered into its kingdom when the commercial dynamo and motor were
first produced. The intrinsic difficulties which electric traction
engineers had to meet were not serious. Designers passed, step by
step, from the model electric railway at the Berlin Exhibition to
public lines on a larger scale, and from the model electric overhead
tramway to the 'street railway' or tramway which gradually supplanted
the horse tramway. Each step consisted in an extension of the distance
covered and an increase in the power required, coincident with a
gradual improvement in the details of motors, dynamos, and transmission
equipment.