Tuning radio with variometers (variable inductance) was before variable capacitors became common. Usually two coils connected in series with inner coil turning inside larger diameter outer coil. This is linear sliding design in this design.
Linear design of coils has larger coil sliding over top of two smaller coils to allow tuning. Construction of sliding variometer is plastic pipe 1 inch inside diameter or 1.25 inch outside diameter about 8.5 inches long. Wind number 22 enameled wire 2.25 inches long or 86 turns. Wind lower windings on tube the same amount allowing for finished gap between the windings of 2 inches.
Center movable coil is using 1.5 inch inside diameter plastic pipe or about 1.875 inch outside diameter. This winding is number 22 enameled wire 2 inches long or 74 tightly wound turns.
To mount, cut 1 inch diameter wood dowel about 8.5 inches long and make flat spot on dowel to allow winding wires to run inside 1 inch plastic tube and slide wood dowel inside 1 inch tube. Mount on 9 inch wood base with 90 degree hardware brackets in wood center dowel holding off wood base to allow center winding to slide.
The 1 u ohm resistors is added to run simulator and diode is 1N34.
Antenna has two connections to cover AM broadcast range. The 25 of capacitor can be changed between 25 and 200 pf and if Fahnestock clips are used this is easy matter. If not wind around brass screws. High impedance earphones such as crystal earphones go where the 20 k ohm resistor is located.
Variometers can be used to tune antenna loading coils, coupler devices, or as a matching device.
