The Artisan variable inductors.
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The inductance of an inductor (sometimes called a coil, since it is a coil of wire, and less frequently called a choke) is based on several factors, one of which is permeability, which has to do with the magnetic properties of a material. (This is not a course in physics, so if you want to understand all of that, you'll have to seek elsewhere.)
Each inductor is wound on the center "rung" of an iron form that looks sort of like a three rung ladder. The iron core has a high permeability, thus allowing a large inductance in a relatively small space. The trick to making the inductor variable is that one of the side arms of the "ladder" can be pivoted up and down at one end, which creates a variable air gap in the form at the other end. (See figure below). A spring-loaded screw with a wing nut makes this adjustment possible.
The core of one of the inductors without the coil of wire in place. The "rungs" are vertical in this picture. Note the spring loaded bolt which allows for adjustment. (See below.) The core is actually made from many thin layers of iron. As you can see, the years have not been kind to the frame that holds the core laminations together.
As the gap increases, the overall permeability decreases since air has a MUCH lower permeability than iron. A decrease in permeability means a decrease in inductance, thus an increase in the resonant frequency of the circuit, and the pitch goes up. Decreasing the air gap decreases the frequency in a similar manner.
