Telegraph Facts

• The telegraph was invented in the 1830's.
• A telegraph works by sending pulses of current through a (long) wire to a remote location.
• At the remote location, the received pulses are converted into a form that can be interpreted by a human operator.
• The transmitter is principally just a switch that allows the flow of current when the switch is pressed.
• Traditionally, the receiver produces a ``clicking'' sound when a current pulse is received that can be interpreted by the operator.
• Alternatively, an optical indication may be produced by routing the received signal through a light.
• The original telegraphs use three types of symbols:

  off (space)

  no pulse is transmitted

  short (.)

  a short pulse is transmitted

  long (-)

  a long pulse is sent

• The ``space'' or ``off'' signal is used to mark the end of a transmitted pulse. In other words, between any two pulses there is always some ``space.''
• In 1843, Congress passed a bill appropriating money for the construction of a telegraph circuit between Washington and Baltimore.