Use coils with varying turns and an iron core to explore the currents and voltages in transformers.
Related Demos
Tips
Attach one coil to the headphone out of the radio and one coil to the input of the amplifier. Moving the coils near each other will allow the signal to pass from one to the other. Try different combinations of turns on each end and observe the effect on volume from the amplifier. Now try using the iron transformer core by passing it through both coils (even on opposite sides) and open/close the loop on the transformer core. Observe the effect on volume.
If batteries are needed there are extras in the wall-mounted cabinet (101) next to the red toolbox.