Iron, or other materials, can be used as the core of an electromagnet, to get a stronger field. What verb describes what the iron does? I'm tempted to say that it conducts the field, but I know that's not right.
(It's not a question about the English language, as was given as a reason for closing the question; it's a question about terminology. If it were a question about electrical current, then "conduct" would be the correct term; but it's not, it's about a magnetic field.)
(And answer anywhere you like; in the comments is fine with me.)