Purpose
To provide a low reluctance path that increases the magnetic flux through the coil.
Specifications
Low reluctance, high permeability magnetic material: Welding rod; 0.042" inch diameter copper coated steel
rod, 3 feet long. (Will be cut to length of the coil spool)
Quantity
Get 3-5 lbs. (around 10 rods of 3 feet each).
Tolerances
Rough cut okay. Diameter is not crucial. It could be smaller by 50% or larger by 100%.
Sourcing
Available at your local welding supply store (such as Oxyarc) or hardware store (such as True Value or Ace)
Try also an auto parts store. Look up industrial suppler or welding suppler in yellow pages of phone
book. Phone them; ask for open box item at school project price