Put the inductor, capacitor, and resistor in series with the wave generator. Monitor the voltage across the inductor.  The resonance for a circuit with a 1K resistor, a .022 mF capacitor, and a 121 mH inductor should be at approximately 15.1 kHz.  Insert the iron core which increases the inductance to approximately 530 mH and changes the resonance frequency to approximately 8.1 kHz.  Replace the iron core with the brass core which has slight diamagnetic properties.  The inductance decreases to about 85 mH and the resonance frequency goes up to approximately 19 kHz.