For town supply the current from the power station is led along underground cables to a number of sub-stations, situated in different parts of the town, and generally underground. At each sub-station the current passes through a step-down transformer, which also acts on the principle of the induction coil, but in the reverse way, so that the voltage is lowered instead of being raised. From the transformer the current emerges at the pressure required for use, but it is still alternating current; and if it is desired to have a continuous-current supply this alternating current must be converted. One of the simplest arrangements for this purpose consists of an electric motor and a dynamo, the two being coupled together. The motor is constructed to run on the alternating current from the transformer, and it drives the dynamo, which is arranged to generate continuous current. There is also a machine called a “rotary converter,” which is largely used instead of the motor generator. This machine does the work of both motor and dynamo, but its action is too complicated to be described here. From the sub-stations the current, whether converted or not, is distributed as required by a network of underground cables.
In many parts of the world, especially in America, water power is utilized to a considerable extent instead of steam for the generation of electric current. The immense volume of water passing over the Falls of Niagara develops energy equal to about seven million horse-power, and a small amount of this energy, roughly about three-quarters of a million horse-power, has been harnessed and made to produce electric current for light and power. The water passes down a number of penstocks, which are tubes or tunnels about 7 feet in diameter, lined with brick and concrete; and at the bottom of these tubes are placed powerful water turbines. The falling water presses upon the vanes of the turbines, setting them revolving at great speed, and the power produced in this way is used to drive a series of very large alternating current dynamos. The current is conveyed at a pressure of about 60,000 volts to various towns within a radius of 200 or 300 miles, and it is anticipated that before very long the supply will be extended to towns still more distant. Many other American rivers have been harnessed in a similar way, though not to the same extent; and Switzerland and Norway are utilizing their water power on a rapidly increasing scale. In England, owing to the abundance of coal, little has been done in this direction. Scotland is well favoured in the matter of water power, and it is estimated that the total power available is considerably more than enough to run the whole of the railways of that country. Very little of this power has been utilized however, and the only large hydro-electric installation is the one at Kinlochleven, in Argyllshire. It is a mistake to suppose that water power means power for nothing, but taking things all round the cost of water power is considerably lower than that of steam.
CHAPTER XI
ELECTRICITY IN LOCOMOTION
The electric tramcar has become such a necessary feature of our everyday life that it is very difficult to realize how short a time it has been with us. To most of us a horse-drawn tramcar looks like a relic of prehistoric times, and yet it is not so many years since the horse tram was in full possession of our streets. Strikes of tramway employees are fortunately rare events, but a few have occurred during the past two or three years in Leeds and in other towns, and they have brought home to us our great dependence upon the electric tram. During the Leeds strike the streets presented a most curious appearance, and the city seemed to have made a jump backward to fifty years ago. Every available article on wheels was pressed into service to bring business men into the city from the outlying districts, and many worthy citizens were seen trying to look dignified and unconcerned as they jogged along in conveyances which might have come out of the Ark. On such an occasion as this, if we imagine the electric light supply stopped also, we can form some little idea of our indebtedness to those who have harnessed electricity and made it the greatest power of the twentieth century.
There are three distinct electric tramway systems; the trolley or overhead system, the surface contact system, and the conduit system. The trolley system has almost driven the other two from the field, and it is used almost exclusively throughout Great Britain and Ireland. On the Continent and in the United States the conduit system still survives, but probably it will not be long before the trolley system is universally employed.
The superiority of the trolley system lies in the fact that it is cheaper to construct and to maintain than the other two, and also in its much greater reliability under all working conditions. The overhead wire is not one continuous cable, but is divided into sections of about half a mile in length, each section being supplied with current from a separate main. At each point where the current is fed to the trolley wire a sort of metal box may be seen at the side of the street. These boxes are called “feeder pillars,” and each contains a switch by means of which the current can be cut off from that particular section, for repairing or other purposes. Above the car is fixed an arm provided with a trolley wheel which runs along the wire, and this wheel takes the current from the wire. From the wheel the current passes down the trolley arm to the controller, which is operated by the driver, and from there to the motors beneath the car. Leaving the motors it passes to the wheels and then to the rails, from which it is led off at intervals by cables and so returned to the generating station. The current carried by the rails is at a pressure of only a few volts, so that there is not the slightest danger of shock from them. There are generally two electric motors beneath the car, and the horse-power of each varies from about fifteen to twenty-five.
The controller consists mainly of a number of graduated resistances. To start the car the driver moves a handle forward notch by notch, thus gradually cutting out the resistance, and so the motors receive more and more current until they are running at full speed. The movement of the controller handle also alters the connexion of the motors. When the car is started the motors are connected in series, so that the full current passes through each, while the pressure is divided between them; but when the car is well on the move the controller connects the motors in parallel, so that each receives the full pressure of the current.
The conduit and surface contact systems are much the same as the trolley system except in the method of supplying the current to the cars. In the conduit system two conductors conveying the current are placed in an underground channel or conduit of concrete strengthened by iron yokes. The top of the conduit is almost closed in so as to leave only a narrow slot, through which passes the current collector of the car. This current collector, or “plough” as it is called, carries two slippers which make contact with the conductors, and thus take current from them. In this system the current returns along one of the conductors, so that no current passes along the track rails. This is the most expensive of the three systems, both in construction and maintenance.
The surface contact or stud system is like the conduit system in having conductors placed in a sort of underground trough, but in this case contact with the conductors is made by means of metal studs fixed at intervals in the middle of the track. The studs are really the tops of underground boxes each containing a switch, which, when drawn up to a certain position, connects the stud to the conductors. These switches are arranged to be moved by magnets fixed beneath the car, and thus when the car passes over a stud the magnets work the switch and connect the stud to the conductors, so that the stud is then “alive.” The current is taken from the studs by means of sliding brushes or skates which are carried by the car. The studs are thus alive only when the car is passing over them, and at all other times they are dead, and not in any way dangerous.
The weight and speed of electric cars make it important to have a thoroughly reliable system of brakes. First of all there are ordinary mechanical brakes, which press against the wheels. Then there are electro-magnetic slipper brakes which press on the rails instead of on the wheels of the car. These brakes are operated by electro-magnets of great power, the current necessary to excite the magnets being taken from the motors. Finally there is a most interesting and ingenious method of regenerative control. Before a car can be stopped after it has attained considerable speed a certain amount of energy has to be got rid of in some way. With the ordinary mechanical or electro-magnetic brakes this energy is wasted, but in the regenerative method it is turned into electric current, which is sent back into the circuit. If an electric motor is supplied with mechanical power instead of electric current it becomes a dynamo, and generates current instead of using it. In the regenerative system, when a car is “coasting” down a hill it drives the wheels, and the wheels drive the motors, so that the latter become dynamos and generate current which is sent back to the power station. In this way some of the abnormal amount of current taken by a car in climbing a hill is returned when the car descends the hill. The regenerative system limits the speed of the car, so that it cannot possibly get beyond control.