A hundred years ago this month, the March 1923 issue of Popular Mechanics carried this self-explanatory idea for a homemade variable capacitor. A glass test tube serves as the dialectric, and the “rotor” is a brass tube sized to just fit over the tube. The stator is foil lining the inside of the tube.

With the dimensions shown here, the capacitance was said to be about 500 pF, which would prove useful for many radio circuits.