Abstract :
During the past twenty years overhead trolley construction for electric railways has become such a familiar type of engineering construction, and apparently so settled in practically all its detail, that it is apt to be regarded as commonplace compared with many other features of electric railway equipment engineering. As long as the electric energy for train propulsion was everywhere standardized at 500 to 600 volts direct current, trolley construction practice followed a comparatively well settled and practically uniform line of standards; but since 1904 wide departures have been made from the original forms of railway motors, both as to current and voltage. This advance, made entirely in the interest of economy in transmission, has been accompanied by radical changes in trolley construction which have required the closest engineering attention and have lifted it above the level of the commonplace into which it had generally come to be relegated.
