[Footnote 1: A paper recently read before the Society of Arts,
London.]
By DR. EDWARD HOPKINSON.
In the summer of 1881, Mr. W.A. Traill, late of H.M. Geological
Survey, suggested to Dr. Siemens that the line between Portrush and
Bushmills, for which Parliamentary powers had been obtained, would be
suitable in many respects for electrical working, especially as there
was abundant water power available in the neighborhood. Dr. Siemens at
once joined in the undertaking, which has been carried out under his
direction. The line extends from Portrush, the terminus of the Belfast
and Northern Counties Railway, to Bushmills in the Bush valley, a
distance of six miles. For about half a mile the line passes down the
principal street of Portrush, and has an extension along the Northern
Counties Railway to the harbor. For the rest of the distance, the
rails are laid on the sea side of the county road, and the head of the
rails being level with the ground, a footpath is formed the whole
distance, separated from the road by a curbstone. The line is single,
and has a gauge of three feet, the standard of the existing narrow
gauge lines in Ulster. The gradients are exceedingly heavy, as will be
seen from the diagram, being in parts as steep as 1 in 35. The curves
are also in many cases very sharp, having necessarily to follow the
existing road. There are five passing places, in addition to the
sidings at the termini and at the carriage depot. At the Bushmills
end, the line is laid for about 200 yards along the street, and ends
in the marketplace of the town. It is intended to connect it with an
electrical railway from Dervock, for which Parliamentary powers have
already been obtained, thus completing the connection with the narrow
gauge system from Ballymena to Larne and Cushendall. About 1,500 yards
from the end of the line, there is a waterfall on the river Bush, with
an available head of 24 feet, and an abundant supply of water at all
seasons of the year. Turbines are now being erected, and the necessary
works executed for employing the fall for working the generating
dynamo machines, and the current will be conveyed by means of an
underground cable to the end of the line. Of the application of the
water power it is unnecessary to speak further, as the works are not
yet completed. For the present, the line is worked by a small
steam-engine placed at the carriage depot at the Portrush end. The
whole of the constructive works have been designed and carried out by
Mr. Traill, assisted by Mr. E.B. Price.
The system employed may be described as that of the separate
conductor. A rail of T-iron, weighing 19 pounds to the yard, is
carried on wooden posts, boiled in pitch, and placed ten feet apart,
at a distance of 22 inches from the inside rail and 17 inches above
the ground. This rail comes close up against the fence on the side of
the road, thus forming an additional protection. The conductor is
connected by an underground cable to a single shunt-wound dynamo
machine, placed in the engine shed, and worked by a small agricultural
steam engine of about 25 indicated horse power. The current is
conveyed from the conductor by means of two springs, made of steel,
rigidly held by two steel bars placed one at each end of the car, and
projecting about six inches from the side. Since the conducting rail
is iron, while the brushes are steel, the wear of the latter is
exceedingly small. In dry weather they require the rail to be slightly
lubricated; in wet weather the water on the surface of the iron
provides all the lubrication required. The double brushes, placed at
the extremities of the car, enable it to bridge over the numerous
gaps, which necessarily interrupt the conductor to allow cart ways
into the fields and commons adjoining the shore. On the diagram the
car is shown passing one of these gaps: the front brush has broken
contact, but since the back brush is still touching the rail, the
current has not been broken. Before the back brush leaves the
conductor, the front brush will have again risen upon it, so that the
current is never interrupted. There are two or three gaps too broad to
be bridged in this way. In these cases the driver will break the
current before reaching the gap, the momentum of the car carrying it
the 10 or 12 yards it must travel without power.
The current is conveyed under the gaps by means of an insulated copper
cable carried in wrought-iron pipes, placed at a depth of 18 inches.
At the passing places, which are situated on inclines, the conductor
takes the inside, and the car ascending the hill also runs on the
inside, while the car descending the hill proceeds by gravity on the
outside lines.
From the brushes the current is taken to a commutator worked by a
lever, which switches resistance frames placed under the car, in or
out, as may be desired. The same lever alters the position of the
brushes on the commutator of the dynamo machine, reversing the
direction of rotation, in the manner shown by the electrical hoist.
The current is not, as it were, turned full on suddenly, but passes
through the resistances, which are afterward cut out in part or
altogether, according as the driver desires to run at part speed or
full speed.
From the dynamo the current is conveyed through the axle boxes to the
axles, thence to the tires of the wheels, and finally back by the
rails, which are uninsulated, to the generating machine. The conductor
is laid in lengths of about 21 feet, the lengths being connected by
fish plates and also by a double copper loop securely soldered to the
iron. It is also necessary that the rails of the permanent way should
be connected in a similar manner, as the ordinary fish plates give a
very uncertain electrical contact, and the earth for large currents is
altogether untrustworthy as a conductor, though no doubt materially
reducing the total resistance of the circuit.
The dynamo is placed in the center of the car, beneath the floor, and
through intermediate spur gear drives by a steel chain on to one axle
only. The reversing levers, and also the levers working the mechanical
brakes, are connected to both ends of the car, so that the driver can
always stand at the front and have uninterrupted view of the rails,
which is of course essential in the case of a line laid by the side of
the public road.
The cars are first and third class, some open and some covered, and
are constructed to hold twenty people, exclusive of the driver. At
present, only one is fitted with a dynamo, but four more machines are
now being constructed by Messrs. Siemens Bros., so that before the
beginning of the heavy summer traffic five cars will be ready; and
since two of these will be fitted with machines capable of drawing a
second car, there will be an available rolling stock of seven cars. It
is not intended at present to work electrically the portion of the
line in the town at Portrush, though this will probably be done
hereafter; and a portion, at least, of the mineral traffic will be
left for the two steam-tramway engines which were obtained for the
temporary working of the line pending the completion of the electrical
arrangements.
Let us now put in a form suitable for calculation the principles with
which Mr. Siemens has illustrated in a graphic form more convenient
for the purposes of explanation, and then show how these principles
have been applied in the present case.
Let L be the couple, measured in foot-pounds, which the dynamo must
exert in order to drive the car, and _w_ the necessary angular
velocity. Taking the tare of the car as 50 cwt., including the weight
of the machinery it carries, and a load of twenty people as 30 cwt.,
we have a gross weight of 4 tons. Assume that the maximum required is
that the car should carry this load at a speed of seven miles an hour,
on an incline of 1 in 40. The resistance due to gravity may be taken
as 56 lb. per ton, and the frictional resistance and that due to other
causes, say, 14 lb. per ton, giving a total resistance of 280 lb., at
a radius of 14 inches. The angular velocity of the axle corresponding
to a speed of seven miles an hour, is 84 revolutions per minute. Hence
L = 327 foot pounds, and _w_ = (2[pi] × 84) / 60.
If the dynamo be wound directly on the axle, it must be designed to
exert the couple, L, corresponding to the maximum load, when revolving
at an angular velocity, w, the difference of potential between the
terminals being the available E.M.F. of the conductor, and the current
the maximum the armature will safely stand. This will be the case in
the Charing-cross Electrical Railway. But when the dynamo is connected
by intermediate gear to the driving wheels only, the product of L and
_w_ remains constant, and the two factors may be varied. In the
present case L is diminished in the ratio of 7 to 1, and _w_
consequently increased in the same ratio. Hence the dynamo, with its
maximum load, must revolve at 588 revolutions per minute, and exert a
couple of forty-seven foot-pounds. Let E be the potential of the
conductor from which the current is drawn, measured in volts, C the
current in amperes, and E1 the E.M.F. of the dynamo. Then E1 is
proportional to the product of the angular velocity, and a certain
function of the current. For a velocity [omega], let this function be
denoted by _f_(C). If the characteristic of the dynamo can be drawn,
then _f_(C) is known.