Complex sentence: So long as the sound was a musical tone it proved efficient, for a musical tone is a regular succession of vibrations.The vibrations of speech are irregular and complicated, and in order to transmit them the current has to be varied in strength without being altogether broken.
Split into simpler sentences: So long as the sound was a musical tone it proved efficient, for a musical tone is a regular succession of vibrations. But the vibrations of speech are irregular and complicated, and in order to transmit them the current has to be varied in strength without being altogether broken.