BY MONTAGUE RHYS JONES, C.E. 81 



With regard to the overhead work a three-tenths of an 

 inch copper wire is stretched over the centre of the track. At 

 every 120 feet it is suspended about 20 feet above the level of 

 the street. This is done either by means of a bracket, as 

 shown on the diagram (Plate X.), or from a thin 

 cable stretched across the street from kerb to 

 kerb, attached to poles either of wood or iron. This, of 

 course, necessitates the insulation of the main wire by means 

 of a small insulator made of mica and indiarubber. 



The poles are planted in concrete to a depth of 6 or 8 feet 

 and a good earth connection made, so that leakage of current 

 can be grounded. The poles are very ornamental in design, 

 and are capable not only of any amount of artistic embellish- 

 ment, but can be practically used for electric lighting. On 

 short roads the overhead wire provides current for the whole 

 of the motors on the track, but on long lines, where traffic is 

 heavy, feeders are resorted to, carried overhead or under- 

 ground, aud connected with the main overhead wire at 

 intervals of 400 to 500 feet by small lateral or sub-feeders. 

 The important features of this excellent system are the 

 maintenance of a constant potential along the entire length 

 of the road, avoiding a breakdown of the system in case of 

 fires, congested street traffic, or the rupture of the overhead, 

 lines. Traffic would still be carried on at either side to the 

 breach, or if by placing cross-over roads in the track at the 

 ends of these sections the inconvenienee would be slight. 



The next point to be explained is the motor and the connec- 

 tion between the motor and the axle of the car. (Plate XI. ) You 

 already know the main functions of the motor. The current 

 is conducted to the motor under the floor of the car. The 

 mechanism consists of a motor fixed under frame of car. On 

 the end of the motor shaft a pinion gears into a spur wheel 

 on an intermediate shaft. On the other end of the latter 

 another pinion gears into a spur wheel on the driving axle of 

 the car, and motion thereby being communicated. The 

 Sprague motors are perfectly automatic, running at nearly 

 the same speed for all roads up to the maximum, and adjusts 

 itself to normal speed under sudden changes in load. A 

 diagram showing a street railway current curve gives an idea 

 of the erratic power-calls on the motor. (PlateXII.) As regards 

 the system of braking, when a motor is in operation it is gene- 

 rating an electro-motive force. In other words, it is acting 

 like a dynamo, and since this depends upon the strength of 

 the field magnet and the intensity of motion, and since the 

 field magnet strength is under positive control, it follows that 

 the electro-motive force can be made to equal the initial 

 motive force, and even to exceed it when this electro-motive 

 force of the motor thus predominates. The machine will 



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