Temperature reference apparatus

Claim 1

IN COMBINATION, A SUBSTANTIALLY THERMALLY INSULATED ENCLOSURE, A TEMPERATURE SENSOR IN SAID ENCLOSURE FOR SENSING VARIATIONS IN TEMPERATURE IN SAID ENCLOSURE, A HEATER FOR INTRODUCING HEAT TO SAID ENCLOSURE, AND CIRCUIT MEANS COUPLED TO SAID SENSOR THROUGH SAID ENCLOSURE AND TO SAID HEATER FOR PROVIDING PULSES TO SAID HEATER AT A CONSTANT FREQUENCY AND AT A DUTY CYCLE CONTROLLED BY SAID SENSOR, SAID CIRCUIT MEANS INCLUDING A SWITCHING ELEMENT SERIALLY CONNECTED WITH SAID HEATER, SAID SWITCHING ELEMENT HAVING A NORMAL HIGH IMPEDANCE CONDITION, AND AN OPERATIVE LOW IMPEDANCE CONDITION, A SOURCE OF UNIDIRECTIONAL SINUSOIDAL PULSES OF CURRENT CONNECTED TO THE SERIALLY CONNECTED HEATER AND SWITCHING ELEMENT, AND A TIMING CIRCUIT INCLUDING SAID SENSOR COUPLED TO SAID SOURCE AND TO SAID ELEMENT FOR DETERMINING THE DURATION OF OPERATION OF SAID ELEMENT DURING EACH PULSE OF CURRENT FROM SAID SOURCE IN ACCORDANCE WITH TEMPERATURE SENSED BY SAID SENSOR.