Gas leak detector

Claim 1

A LEAK DETECTOR COMPRISING AN ELEMENT RESPONSIVE TO THE PRESENCE OF GASEOUS SUBSTANCES FOR PRODUCING D.-C. SIGNALS REPRESENTATIVE OF THE CONCENTRATION OF SAID GASEOUS SUBSTANCES, MEANS COUPLED TO SAID ELEMENT CAPABLE OF ASSUMING A PLURALITY OF DIFFERENT OPERATIVE CONDITIONS FOR INTRODUCING DIFFERENT LEVELS OF ATTENUATIONS IN SIGNALS APPLIED THERETO IN ADDITION TO A ZERO LEVEL OF ATTENUATION, D.-C. SIGNAL AMPLIFYING MEANS COMPRISING INPUT AND OUTPUT CIRCUITS AND MEANS CAPABLE OF ASSUMING A PLURALITY OF DIFFERENT OPERATIVE CONDITIONS FOR CONTROLLING THE GAIN OF SAID AMPLIFYING MEANS, MEANS FOR COUPLING THE OUTPUT SIGNALS OF SAID ATTENUATION MEANS ACROSS THE INPUT CIRCUIT OF SAID SIGNAL AMPLIFYING MEANS, A BRIDGE CIRCUIT HAVING SAID OUTPUT CIRCUIT CONNECTED AS ONE LEG THEREOF, SAID OUTPUT CIRCUIT COMPRISING RESISTIVE MEANS RESPONSIVE TO SIGNALS APPLIED THERETO FOR PRESENTING VALUES OF RESISTANCE PROPORTIONAL TO THE MAGNITUDE OF THE APPLIED SIGNALS, UTILIZATION MEANS CONNECTED ACROSS ONE DIAGONAL OF SAID BRIDGE CIRCUIT, A FIRST SOURCE OF POTENTIAL CONNECTED ACROSS THE OTHER DIAGONAL OF SAID BRIDGE CIRCUIT, AND MEANS OPERATIVE FOR SELECTIVELY CONTROLLING LEAK DETECTOR SENSITIVITY BY SELECTIVELY CONTROLLING THE OPERATIVE CONDITION OF BOTH SAID ATTENUATING MEANS AND SAID GAIN CONTROLLING MEANS TO THEREBY PROVIDE A LEAK DETECTOR HAVING A PLURALITY OF DIFFERENT SELECTABLE VALUES OF SENSITIVITY.