The Portrush electric railway extends from Portrush a distance of 6 miles. The line is single, and has a gauge of 3 ft. The gradients are exceedingly heavy, 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 5 passing places, in addition to the sidings at the termini and at the carriage depot. About 1500 yd. from the end of the line is a waterfall on the river Bush, with an available head of 24 ft., and an abundant supply of water at all seasons of the year. Turbines are being erected 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. At present the line is worked by a small steam engine.
The system employed may be described as that of the separate conductor. A rail of T - iron, weighing 19 lb. to the yd., is carried on wooden posts, boiled in pitch and placed 10 ft. apart, at a distance of 22 in. from the inside rail and 17 in. above ground. The conductor is connected by an underground cable to a single shunt - wound dynamo machine, worked by a small agricultural steam - engine of about 25 I.H.P. The current is conveyed from the conductor by means of 2 steel springs, rigidly held by 2 steel bars placed one at each end of the car, and projecting about 6 in. 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 car passing one of these gaps, the front brush breaks contact, but since the back brush still touches the rail, the current is not broken: Before the back brush leaves the conductor, the front brush will have again risen upon it, so that the current is never interrupted. When gaps are too broad to be bridged in this way, the driver breaks the current before reaching the gap, the momentum of the car carrying it the 10 or 12 yd. 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 in. 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. The current is not, as it were, turned full on suddenly, but passes through the resistances, which are afterwards 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 tyres 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 ft., the lengths being connected by fish - plates and 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 centre 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 the levers working the mechanical brakes, are connected to both ends of the car, so that the driver can always stand in 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.
For calculation, let L be the couple, measured in foot - lb., 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 20 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 7 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 fric - tional resistance and that due to other causes, say, 14 lb. per ton; giving a total resistance of 280 lb., at a radius of 14 in. The angular velocity of the axle, corresponding to a speed of 7 miles an hour, is 84 rev. per minute. Hence
L = 327 foot - lb., and w =2π x 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 to, 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. But when the dynamo is connected by intermediate gear to the driving wheels only, the product of L and w remains constant, and the 2 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 make 588 rev. per minute, and exert a couple of 47 foot - lb. 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 Ώ, let this function Be denoted by /(C). If the characteristic of the dynamo can be drawn, then /(C) is known.