Complex sentence: Then, by simply placing a carbon microphone in the transmission line, the strength of the signal could be varied in order to add sounds to the transmission--in other words, amplitude modulation would be used to impress audio on the radio frequency carrier wave.
Split into simpler sentences: Then, by adding a simple carbon microphone in the transmission line, the strength of the signal could be varied in order to add sounds to the transmission. In other words, amplitude modulation would be used to impress audio on the radio frequency carrier wave.