How does it work?
To use the radio, connect one end of the alligator jumper to your antenna. Connect the other end to a good ground, such as a cold water pipe.
We will start the tuning with the variable capacitor set in the middle of its range, neither all the way clockwise, nor all the way counter clockwise.
With the earphone in your ear, slowly move the ferrite rod into the coil, listening for radio stations.
With a long antenna, and a single loop of the alligator jumper, you can tune several radio stations. In some areas, one or two stations will be so close or so powerful that they overwhelm all the others, and you will only hear those one or two stations.
If you have a shorter antenna, the stations will sound very faint if you only use one loop of the alligator jumper. Looping the jumper around the coil two or three times will increase the volume. But the volume increase comes at a price -- you will hear fewer stations.
The number of loops in the alligator jumper trade off volume for selectivity. The best way to increase the volume is to use as long an antenna as you can, and a good ground connection.
How does the ferrite change the frequency?
The ferrite rod increases the inductance of the coil. In our other (hand-wound) coils, we increased the inductance by winding some more loops, or by using a "tapped" coil, and selecting a tap that was farther down the coil.
As the ferrite rod is inserted into the coil, more of the coil is affected by the ferrite, and so the inductance increases. Increasing the inductance moves the frequency lower. This allows us to hear stations "lower on the radio dial".
Ferrite is used because it is magnetic, like iron or steel, but it is not a conductor of electricity. If it were conductive, the coil would induce "eddy currents" in it, and some of the energy would be lost heating up the core. Because ferrite is not a conductor, we can use its magnetic properties to change the inductance of the coil, without losing volume.
If you have a long antenna, a good ground, and you are not too close to a strong station, the variable capacitor will help in fine tuning the stations.
