Temperature-sensing circuit

A method for intermittently interrogating a temperature-sensing circuit to verify the operative condition thereof at ambient temperatures which may be different from the in-situ ambient temperature during interrogation, said circuit comprising an in-situ reed switch and magnetic means associated therewith, including temperature-responsive ferrite means in magnetic circuit with the reed switch for establishing a magnetic field in the vicinity of the reed switch contacts, whereby said reed switch is operable, under the influence of the magnetic field, between open and closed positions at ambient temperatures above and below the Curie point temperature of the magnetic circuit, said method comprising the steps of selectively altering said magnetic field by an amount effective to establish temporarily a false Curie point temperature causing a reed switch to change from one to the other of said positions, and providing an indication of the position of said reed switch prior and subsequent to the alteration of the magnetic field.