A crystal radio receiver circuit with detector bias battery and testing buzzer. This type receiver circuit was used with cat's whisker detectors during the wireless telegraphy era prior to the 1920s.  Many of the crystals used in early cat's whisker detectors, particularly carborundum (silicon carbide), had large band gaps (forward voltage drops).  These weren't very efficient rectifiers in crystal radios, because the low voltage signal from the antenna had to overcome the device's forward voltage drop, and so was insufficient to drive the device far into its conduction region, so it had a large AC resistance.  To make the detector D1 more sensitive, an adjustable bias voltage equal to the forward voltage drop is applied by battery B1 and potentiometer R1 across the detector.  For silicon carbide about 1 V was used.  R1 was adjusted until the radio station sounded loudest in the earphone.  The bias moved the DC operating point to the most sensitive "knee" of the detector's IV curve
The cat's whisker detector D1 consisted of a fine wire touching the surface of a piece of crystal.  The metal-crystal contact functioned as a primitive PN junction, allowing current to flow in only one direction.  Only certain sites on the crystal surface functioned as rectifying junctions, so an active site had to be found by trial and error before each use.  The operator dragged the wire across the crystal surface until the detector began functioning.  The battery-powered buzzer circuit (B2, BZ, S1) generated a test signal to determine if the detector was functioning.  Switch S1 was closed and the electromagnet-operated buzzer BZ was turned on while the detector was adjusted.  The sparks at the buzzer's contacts acted as a weak radio transmitter, and its signal was applied to the receiver's ground wire.  So when the right spot on the detector was found, the buzz could be heard in the earphones.  Then the buzzer was turned off.
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Chetvorno