olemus
Wiktionary:
1.2. the heart or inner part of a physical thing
3.7. a piece of ferromagnetic material (e.g., soft iron), inside the windings of an electromagnet, that channels the magnetic field.

näiteid
(2a) elektromagneti, trafo vm induktiivkomponendi magnetsüdamik
https://en.wikipedia.org/wiki/Magnetic_core

(2b) valguskaablis: signaali kandev kiu siseosa
https://www.thefoa.org/tech/ref/basic/ljar-fib.jpg