for making contact, and these wheels are just the length of a section apart. When the leading wheel L is on a negative section, as A2, the trailing wheel T is on a positive section, A1, and vice versa, 
Fig. 5.—Mechanical Brake. so that a circuit is made between the poles of the generator through the rails, the two contact wheels of a train, and a wire connecting these wheels through the motor M on the locomotive, which thus receives its supply of electric energy. Of course, the current through the motor is stopped and reversed each time the contact wheels pass from a positive to a negative or a negative to a positive section, but this makes no difference with the direction in which the motor runs, nor does it injure the dynamo. We can cause the locomotive to run backward, however, by altering the positions of the commutator-brushes on the motor. In the case of a single track, the positive sections A1, A3, etc., would have to
Fig. 6.—Diagram of the Cross-over Parallel System.
be connected by a long wire instead of through the sections of rail B2, B4, etc. This system requires that the sections shall all be of equal length, which is sometimes inconvenient, as when broad gorges have to be crossed, and at curved parts of the line. But this difficulty can be overcome to some extent by employing a "gravitation section" longer than the distance between the contact wheels of a train. This section is constructed with a downward slope, so that the weight of the train will propel it over the part in which it receives no electric energy.
