Or it could excite an electro-magnet, which, by attracting a piece of soft iron, would inscribe the passage with a pen or pencil.
(it; could excite; an electro-magnet, which, by attracting a piece of soft iron, would inscribe the passage with a pen or pencil) (an electro-magnet; would inscribe; the passage with a pen or pencil)
it an electro-magnet
could excite would inscribe
an electro-magnet, which, by attracting a piece of soft iron, would inscribe the passage with a pen or pencil
the passage with a pen or pencil