To send voice and music, amplitude modulation had to be developed. And a key ingredient of this is the carrier wave.
The problem is, the carrier wave needs to have somewhat high frequencies, in the hundreds of kHz TODO why. But as you might imagine, that is hard to achieve by mechanical means such as a hand cranck like Hippolyte Pixiis alternator!
Interestingly, some of the first carrier wave generators were actually mechanical, e.g. the Alexanderson alternator.
But clearly such mechanical machines were not very scalable, and soon more electronic devices were introduced, notably the vacuum tube.
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A carrier wave is the rapid reference oscillation in a modulated wave. In the narrow-packet representation , choose near the centre of the frequency spectrum and on its dispersion relation. Its constant-wave phase reference moves at phase velocity . This reference need not coincide with a local maximum of the complete real wavefield when the wave envelope varies. Moving the reference shifts some slow wave phase into without changing .