I have two coils standing next to each other. One is connected to a power supply and the other one is connected to a lamp. When I put an I-shaped iron core through both coils the lamp shines brighter than without the iron core. The same thing happens when I use a U-shaped iron core. When I put an I-shaped iron core on top of the U-shaped iron core the lamp shines much brighter. I think I understand why an iron core makes the lamp shine brighter. What I don't understand is why the lamp shines much brighter when I use a U-shaped iron core with an I-shaped iron core on top of it. What is the difference between on the one hand having only an I-shaped iron core or only a U-shaped iron core and on the other hand having a U-shaped iron core with an I-shaped iron core on top of it? I have been looking for an explanation but I can't find anything. Grateful for help.