electromagnetic waves (rather than some form of wire) carry the
signal over part or all of the communication path. Some monitoring
devices, such as intrusion alarms, employ acoustic waves at
frequencies above the range of human hearing; these are also
sometimes classified as wireless.
The first wireless transmitters went on the air in the early 20th
century using radiotelegraphy (Morse code). Later, as modulation made
it possible to transmit voices and music via wireless, the medium
came to be called "radio." With the advent of television, fax, data
communication, and the effective use of a larger portion of the
spectrum, the term "wireless" has been resurrected.
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