Ferromagnetic material passing through the center of an energised coil will pull the flux inwards. This can generate an output pulse from a secondary coil closer to the center. (Some railguns are driven with a Pulsar which uses a reciprocating piston engine and a superconducting outer coil.). Carbon is diamagnetic and pushes the flux outwards, rather than pulling it inwards. Bifilar, rather than concentric windings might work, as long as one winding is energised.