Jernkerne i en elekromagnet hvordan?

http://da.wikipedia.org/wiki/Elektromagnet

og her er der noget mere kompliceret omkring jernkernen, dog på engelsk.

How the iron core works

The material of the core of the magnet (usually iron) is composed of small regions called magnetic domains that act like tiny magnets (see ferromagnetism). Before the current in the electromagnet is turned on, the domains in the iron core point in random directions, so their tiny magnetic fields cancel each other out, and the iron has no large scale magnetic field. When a current is passed through the wire wrapped around the iron, its magnetic field penetrates the iron, and causes the domains to turn, aligning parallel to the magnetic field, so their tiny magnetic fields add to the wire's field, creating a large magnetic field that extends into the space around the magnet. The larger the current passed through the wire coil, the more the domains align, and the stronger the magnetic field is. Finally all the domains are lined up, and further increases in current only cause slight increases in the magnetic field: this phenomenon is called saturation.
When the current in the coil is turned off, most of the domains lose alignment and return to a random state and the field disappears. However some of the alignment persists, because the domains have difficulty turning their direction of magnetization, leaving the core a weak permanent magnet. This phenomenon is called hysteresis and the remaining magnetic field is called remanent magnetism. The residual magnetization of the core can be removed by degaussing.

En jernkerne forstærker ikke et magnetfelt (fluxen). Den dæmper blot magnetfeltet mindre end f.eks. luft, fordi jern leder en magnetisk flux en faktor ca. 1000 bedre end luft ( og vaccum ).

Du kan sammenligne det med, at har du en spændingkilde på f.eks. 10V, og belaster denne med en modstand på 10000Ω, så for du en strøm gennem modstanden: I = U / R = 10 / 10000 A = 0,001A. Hvis du udskifter modstanden til en modstand på 10Ω, får du en strøm gennem modstanden: I = U / R = 10 / 10 A = 1A. Men 10Ω's modstanden forstærker ikke strømmen, den lader blot strømmen passere med mindre modstand.