It is a Drain Dip Oscillator based upon a Wheatstone bridge circuit; the oscillator being one resistor of the bridge.
Use the pot to set the meter to (about) mid-scale and position the GDO coil beside the tuned circuit you want to test. “Sweep” the GDO tuning until you see a “dip” and read the frequency from the tuning scale. It can be useful to set the frequency to that which you want, then adjust the tuned circuit under test for a dip. In this way it is possible to align a QRP TX or RX before power is applied.
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