Complex sentence: In France, physician Jacques d'Arsonval from 1889 had pioneered applying RF current to the body and documenting its effects, but his resonant spark circuits could not produce the high voltages that Tesla coils could.
Split into simpler sentences: In France, physician Jacques d'Arsonval from 1889 had pioneered applying RF current to the body and documenting its effects. D'Arsonval's resonant spark circuits could not produce the high voltages that Tesla coils could.