Differentiating metal detector for detecting metal objects and distinguishing between detected diamagnetic and non-diamagnetic objects

Claim 1

A METAL DETECTOR COMPRISING: A FIRST AND A SECOND OSCILLATOR, EACH HAVING A SERIES RESONANT TANK CIRCUIT TUNED TO OSCILLATE AT LESS THAN 15,000 CYCLES PER SECOND AND AT A FREQUENCY DIFFERENT FROM EACH OTHER; A MIXER CONNECTED TO SAID OSCILLATORS FOR DERIVING A DIFFERENCE FREQUENCY EQUAL TO THE DIFFERENCE BETWEEN THE FREQUENCIES OF OSCILLATION OF SAID OSCILLATORS; A TUNED AMPLIFIER CONNECTED TO SAID MIXER, SAID TUNED AMPLIFIER INCLUDING A TANK CIRCUIT TUNED TO A PREDETERMINED DIFFERENCE FREQUENCY FOR PROVIDING AN OUTPUT SIGNAL HAVING AN AMPLITUDE PROPORTIONAL TO THE DEVIATION OF SAID DIFFERENCE FREQUENCY FROM SAID PREDETERMINED DIFFERENCE FREQUENCY; INDICATING MEANS CONNECTED TO SAID TUNED AMPLIFIER FOR INDICATING THE AMPLITUDE OF SAID OUTPUT SIGNAL; AND A SEARCH COIL COUPLED TO THE SERIES RESONANT TANK CIRCUIT OF SAID FIRST OSCILLATOR.