Abstract: An apparatus to be connected in combination with a conventional telephone receiver and its circuit which prevents the telephone receiver from being activated (ringing) until a preselected code is supplied into the telephone circuit by the calling party.

Abstract: An apparatus for locating fluid leaks within a fluid channel such as a gas leak within a pipe line. This apparatus detects the leak by detection of sound waves created by the leaking fluid. The apparatus utilizes a sound transducer which picks up the noise of the leak. Only the ultrasonic portion of the noise is desired with the audible range to be eliminated. Eliminating of the audible range is by subtracting a reference signal from the noise produced signal. The resulting output signal is amplified and supplied to an annunciator. The annunciator can take the form of either or both an audio annunciator or a visual annunciator.