The carbon mike needs a good current through it to make it work. Try to
feed it through a resistor calculated for 20mA through the mike (from 12V
or such). The impedance is what you measure across the mike when biased,
in parallel with the bias R, plus a small capacitive parallel component
(<5nF afair). The output signal will be large (1 Vpp is easy at 1" from
speaker's mouth with the indicated bias circuit).

Peter

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