80 ELECTRIC TRACTION". 



parallel wires, and the other only one overhead wire, the rails 

 and earth forming the return current. There are various 

 methods of contact between the car and the overhead wire, 

 but the one universally used is the single wire under contact 

 system, with bearing wires suspended crossways in narrow 

 streets and centre pole suspension in wide streets, the chief 

 points in favour of these arrangements being the neatness 

 and simplicity of construction, the ease and perfection of all 

 switching, and the well designed and permanent attachment 

 of the trolly arm, a balanced and pivoted pole attached to the 

 car rendering the danger of a falling trolly a very remote 

 contingency. (Plate VIII.) 



There are two prominent firms in America who by their 

 great ability and energy have been instrumental in carrying 

 out most of the electric roads in America. I refer to Sprague 

 and Thomson-Houston. There is little in point of difference 

 between the systems of either firm, but I shall describe 

 Sprague' s system as a type of this modern electrical railway, 

 having been in communication with that firm in reference to 

 the design of a plant for an electrical tramway, whose con- 

 ditions were of an exceptionally difficult character, there 

 being a grade of 1 in 8| for a distance of eight chains. 



We have to consider five points in connection with the 

 overhead system : — 1. The power-house or generating 

 station. 2. The conductor from the power-house to the car. 

 3. The motor attached to the car. 4. The connection 

 between the motor and the axle of the car. 5. The return of 

 the current to the power-house. 



Thrown on the screen is one of Sprague' s power-houses 

 (Plate IX.), supplied with Armington and Sims' 

 engines connected up to the dynamos, and thereby 

 supplying the necessary mechanical power for the 

 production of the current, and in designing these 

 engines it is desirable that they should be so constructed 

 that no variation of more than 2 per cent, in the speed should 

 take place, whether there is a small amount of work to do or 

 whether they are called upon to develop their highest 

 capacity at any given moment, and it is always necessary to 

 have the chief parts in duplicate in case of accident. 



The conductor from the power-house to the car has under- 

 gone many modifications, but the plan which is now generally 

 adopted is a trolly wheel, firmly fixed to a balanced and 

 pivoted pole having a universal movement. This projects 

 from the roof of the car, and is kept in contact with the con- 

 ductor or overhead wire by aid of springs from below press- 

 ing it underneath and against the overhead wire. 



