My great friend James sent me the original design link to this circuit, (TechLib.com/electronics "The Curious C-Beeper"). This capacitance detector will detect as little as 3 pF and oscillate at frequencies from kHz to fractional Hz. with capacitors up to about 1 microFarad or so. Leaky capacitors will not work, of course.  

James modified the circuit to function with three NiMh batteries (3.6 to 4.2 Volts) and temperature compensate the bias point with the diode D1. Connect the capacitor between the "Probe" connection and circuit ground. For best results, build the entire thing in a metal box for shielding. The assembled unit will produce a great "siren" sound when you connect a 150 pF variable capacitor and vary it. You can still scrounge such a variable unit from an old AM radio.

I am considering building an "oil dipstick" level measuring transducer with the circuit. Finally, there needs no on/off switch, since the unit draws less than 10 microamps with no capacitance connected to the probe, but you must adjust the variable trimmer C1 to achieve no oscillation with no capacitance attached.