Abstract: A self-contained, hand-held device for testing an aircraft helmet communication system including a microphone and earphones in which a selection switch on the housing of the device is movable from an off position to a talk-through position at which the microphone is connected to both the earphones and a volume indicator on the housing and to a self-test position in which actuation of a pushbutton switch on the housing applies the output of an oscillator both to the volume indicator and a circuit having a voltage divider made up of a potentiometer and a fixed resistance to provide a self-test signal which energizes an illuminable element on the housing if the device is functioning properly and to an earphone impedance test position at which the output of the oscillator is applied to a circuit having a voltage divider made up of the potentiometer and the earphones to produce an earphone impedance-test signal which energizes the illuminable element if the earphone impedance is of the correct value.

Abstract: An overtemperature detection cable for use with a fire alarm system, a fire extinguishing system or the like comprises a coaxial assembly of an inner spring wire conductor in a core of thermoplastic dielectric material within an electrically conductive outer sheath. In one embodiment, the spring wire is initially coiled in a three-dimensional helical configuration, is thereafter uncoiled to approximate a straight wire, and is then disposed within the outer sheath to reside in the thermoplastic dielectric core. In a second embodiment, a straight inner spring wire and its surrounding thermoplastic dielectric core reside within an outer sheath, and the assembled sheath, wire, and thermoplastic dielectric material are then bent or crimped to assume a two-dimensional sinusoidal configuration.