iron core; iron will
Literal meaning: the iron core of an electromagnet or transformer. Figurative meaning: an unyielding, determined spirit.
The iron core of this transformer is made of high-quality silicon steel.
He has an iron will and never gives in to any difficulty.
toroidal core; endless core
will of iron; iron will; unyielding spirit
Compound of 鉄 (iron) and 心 (heart, core). The figurative sense 'iron will' is a straightforward metaphor from the literal 'iron core'.