THE ELECTRIC TROLLEY 259 



Field in Stockbridge, Mass., and Edison in Menlo Park, 

 N. J. In 1883 an electric railway was constructed at the 

 Chicago Railway Exposition. The electricity was conveyed 

 to the motor by means of a central rail. The first com- 

 mercial electric street railway in this country was constructed 

 in 1885 between Baltimore and Hampden, Md., a distance 

 of two miles. Since that time many improvements have 

 been made in the electric car, and it has advanced with 

 rapid strides, until now the trolley is a common sight in all 

 parts of the United States. A few figures will give some 

 idea of this wonderful growth. In 1885 there was one electric 

 railway in the United States; in 1890 there were 144; in 

 1915 there were 1027 electric railway companies, 46,500 

 miles of track, and 79,000 passenger cars. 



The mechanism of the trolley car. We may now attempt 

 to understand some of the principles that underlie the work- 

 ing of the trolley car. If we take into account all the de- 

 tails, it is a very complicated piece of mechanism, but some 

 of the simpler facts regarding it can be easily understood. 

 As electricity is needed to run the trolley car, let us first 

 notice how this electricity is made. This takes us first to 

 the power house. 



Dynamo. The dynamo is the machine by means of which 

 electricity is generated. It consists of two essential parts, 

 an armature and a pair of field magnets. The armature 

 consists of a coil, or series of coils, of wire and is placed be- 

 tween two field magnets. The armature is so mounted on 

 an axle that it rotates between the magnets. Magnets 

 have power to attract iron, and if iron filings are scattered 

 over a bar magnet they will arrange themselves in regular 

 lines. When the armature revolves between the magnets, 

 it is cutting across the lines of force running out from these 

 magnets, and as a result an electric current is set up' along 

 the wire. By means of proper connections this current may 

 be led out and carried away by wires. Two kinds of current 



