What is this object? (Magnetic/electric problem)

[Greywolfe1982]

The base of the object is a fairly simple electrical circuit - it's plugged into an electrical outlet for a power source, and there is a circuit with a knife switch, a lightbulb and a coil of copper wire (700 turnings, if it makes a difference). Surrounding the copper wire is 3 iron bars, positioned in an upside-down-U shape. Here's a rough sketch of it:

http://img689.imageshack.us/img689/3163/83145558.png

When the switch is on, the light bulb will turn on, however how bright it is depends on the positioning of the iron bars: when the iron bars are far above the coil of copper wire, the light bulb is bright, but when they are brought down to surround the top/left/right of the coil (the areas in front of the front, back and bottom do not change), the light dims. I need to explain what is causing this.

My thought was that it was that the iron became magnetized from the current running through the copper wire, causing the domains to line up in a certain direction (http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html#c4). The magnetic field from the iron then affects the current flowing through the circuit (Faraday's Law) which makes the light bulb become more or less bright. I know this isn't right though...can anybody help me out, or give me the name of the object so I can research it?

 

[Greywolfe1982]

Aaaanybody?

 

[Dick]

It's an inductor. The iron is a imperfect conductor so the changing magnetic fields induce currents and 'magnetic friction' leading to energy loss and heating in the iron. Look up things like "eddy currents" and "hysteresis".