halves of the conductor instead of lodging on it, as would be the case if a single conductor were placed directly under the slot. The collector consists of a steel frame narrow enough to pass through the slot and of contacts sliding along the underground conductor. The contacts are insulated from the steel frame, and are in electrical communication with a clip terminal on the car by means of an insulated cable. Light leather straps serve to draw the collector along in the slot. Should an obstruction occur in the slot or in the conductor of so serious a nature that it cannot be brushed away by the passage of the collector, the latter is arrested and the leather straps break. The strain next comes on to the insulated cable, which is thereby drawn out of the clip terminal, and thus the current is interrupted and the car comes to rest. In this manner the attention of the driver is called to the obstruction, which then can be removed by hand. From the terminal at the under-side of the car the current is led through a variable resistance and a reversing switch to the motor, and returns through the wheels to the rails, and along them back to the generating station. At first the motors were shunt-wound, so as to avoid racing when the car was lightly loaded and running on a level part of the line, or heavily loaded and running down an incline. It has been explained on page 133 that the speed of a shunt motor, when running light, can never exceed a certain limit, whereas a series motor may, under the same condition, assume a dangerously high speed. On purely theoretical grounds shunt motors are, therefore, more suitable for tramway work. But a serious practical difficulty was soon encountered. It arose from the uncertainty of electrical contact between the wheels and the rails. When a current of electricity has to pass through

UNCERTAINTY OF SLIDING CONTACT.

295

two pieces of metal in contact, the first condition is that the surfaces should be clean, and that is precisely the condition which cannot always be fulfilled in a tramway exposed to the weather, and overrun by other traffic. It would thus occasionally happen that the current was interrupted for a very short time, perhaps only a fraction of a second, but the interval was sufficient to cause the field of the motor to lose its magnetism. The consequence of this was, that when contact was restored and the current began again to flow, the armature was not able to offer any counter-electro-motive force, and an abnormal rush of current took place before the field magnets had had time to again become excited. It will be noticed that the injurious effect here described will be the greater the lower the resistance of the armature-that is to say, the more efficient the motor, the more will it suffer from

an occasional interruption of current. Since it was impossible to absolutely avoid these interruptions, the use of shunt motors was discontinued, and series motors were substituted. In a series motor the intensity of the field and, therefore, the counter-electro-motive force of the armature are at once restored when the current begins to flow, and no abnormal rush of current can take place. To prevent racing when lightly loaded, variable resistances placed below the platform at either end of the car have to be used. These resistances are also employed for regulating the speed when the motor is doing a fair amount of work. The use of artificial resistances in this case a necessary adjunct of the system entails, of course, some waste of energy, and in this respect Mr. Reckenzaun's method of varying the power by a combination of motors is preferable. The motors-one to each of the six cars now in use-are 6 horse-power nominal, but may

All

for a short time be worked up to 10 horse-power, the speed being 1,000 revolutions a minute. Each motor weighs 9 cwt. It is worked at an average speed of 800 revolutions a minute, and requires an average current of 18 amperes at 200 volts pressure, or about 5 electrical horse-power, equal to 4 brake horse-power, to propel a car with 45 passengers on a level road. The direction of motion is reversed electrically by reversing the direction of the current through the armature, but not through the field magnets. In doing this the brushes are not shifted, and the diameter of commutation remains always at right angles to the magnetic axis of the field. The brushes consist of small solid blocks of copper, pressed by springs very tightly against the commutator. screws are of steel, and provided with lock-nuts to stand the vibration of the car without becoming loose. The armatures are 10 in. in diameter, and wound with a single layer of 63 mils wire, insulated with pure silk. The generators, of which there are two, placed in a generating station at about the middle of the line, are of the type described on page 262 and illustrated in Fig. 95. Each of these dynamos weighs 4 tons, and is capable of delivering a current of 180 amperes at 300 volts pressure, when worked at a speed of 500 revolutions a minute. But since it was found that a pressure of 200 volts is sufficient to work the present traffic, the speed has been reduced to 350 revolutions a minute. The armature is 16 inches in diameter, and the field magnets, which are of the fourpole type, are separately excited by small dynamos (illustrated in Fig. 93), for the purpose of being able conveniently to alter the electro-motive force within certain limits, according to the requirements of the service.

The Telpher Line at Glynde is an electric railway

about a mile long, acting automatically, without assistance of guard or driver, and intended for the conveyance of a continuous stream of light vehicles, suspended from and rolling on a single line of rails, which at the same time form the electric conductors. In the illustration (Fig. 111), M is the telpher locomotive, consisting of an electro-motor, chain gear, and driving wheels with indiaFig. 111.

rubber treads, and also provided with two governors. One of these breaks the current when the speed attains a certain limit, and the other puts a brake on when the speed should, on a downward gradient, be still further increased. On either side of the locomotive there are placed 5 skeps, each weighing 101 lbs., and capable of carrying 250 to 300 lbs. of clay, and these skeps are kept

the right distance apart by connecting rods. The total length of the train, consisting in all of 11 vehicles, is exactly equal to the distance between two poles, and since the sections of the rail attached to the poles form alternately the out-and-home conductor, it follows that the first and last skep are at all times in contact with rails of opposite polarity. The current is collected at the two ends of the train, and conveyed along wires (not shown in the illustration) to the middle, where it works the motor, M. The arrangement of the circuit is shown diagrammatically in Fig. 112, where D is the generating dynamo, and L, T, L1, T1, two trains, one up and the other down the line. The sections, A1, B2, A3, B4, A5, and so on, are

29

connected together by cross-connections shown in dotted lines, and are also connected with the positive terminal of the dynamo, whilst the alternate sections, B1, A2, B3, A ̧, and so on, are similarly connected, and are also connected with the negative terminal of the dynamo. The conductor is formed of steel rods, inch in diameter, 66 feet long between the poles, and placed 8 feet apart. The motor is regulated to run at a speed of 1,600 to 1,700 revolutions a minute, the speed of the train being 4 to 5 miles an hour. One train running backwards and forwards will deliver 150 tons of clay per week, but as many as 20 trains can be run on the double line at the same time. To avoid in this case the risk of collision, the late Professor Fleming Jenkins, in conjunction with Pro