BLOCK SIGNALS FOR ELECTRIC RAILWAYS.
The simplest block signal used by electric roads is a hand-operated one constructed on the principle shown in the diagram [Fig. 85]. A double throw switch is placed at each terminal of the section of track that is to be protected.
Fig. 85.
The switches have no central position, the knife blade always making contact with one or the other of the terminals shown. If the lamps are lighted, throwing either one of the switches will put them out. If they are not burning, they will be lighted by throwing either one of the switches.
A motorman on reaching a section of track finding the lamps not burning throws the switch. Lamps now burn in each switch box and show that the section is in use. On arriving at the other terminal of the block the switch is thrown, extinguishing the lights and showing that the block is clear.
Automatic signal systems have been devised on the same principle, in which magnets, operated by contacts made by the passage of the trolley wheel, cause the lamps to be lighted and extinguished automatically.