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Up: The TelegraphSeptember 14, 1998
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- The telegraph was invented in the 1830's.
- A telegraph works by sending pulses of current through a (long) wire to a
- 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.
Prof. Bernd-Peter Paris