A Variometer Crystal Set

A variometer is an electronic component consisting of two inductors (coils) connected in series, and physically arranged such that the coupling between the inductors can be varied.  This causes the inductance of the total device to be variable over a considerable range. 

There is often enough self capacitance that a variometer can be used as a wide-range parallel tuned circuit.

1920's Remler variometer made of molded Bakelite.

A variomenter can be used to build a simple xtal set.  However, typical of 99% of published schematics, this is not a great circuit.

In 1921 Westinghouse Electric Corporation  introduced a low cost crystal set in support of their pioneering radio station KDKA.  This set was designed by real radio engineers, and works quite well, considering its simplicity.

Keep in mind, thei is city-mouse territory, you can drive a speaker easily here in Jersy City, NJ.

Here's my Aeriols Junior clone using vintage parts.  This is typical of 1920's homebrew using thumb nuts and Fanestock clips,  I did have to make one solder joint.

Like I said, these guys knew what they were doing.  To provide an optimal match to the detector, they made a tap half way up the stator.  I had to locate the connection between the two stator coils, and make ny tab there.

See it in an IEEE Video.