Ionization fire alarm with wind screen

Claim 1

An ionization fire Alarm, comprising at least one ionization chamber to which the external air has access, said ionization chamber including at least one radioactive substance and two electrodes, said ionization chamber being connected in series with a resistance element and across two conductor lines supplied by a D.C. voltage which is less than 60 volts, the connecting point of said ionization chamber and said resistance element being connected to a gate electrode of a field-effect transistor, said ionization chamber further including sheathing means for reducing the velocity of air flow within said ionization chamber, said sheathing means comprising a respective outer and inner sheathing portion, said outer and said inner sheathing portions being separable from each other, the inner surface of said outer portion having a geometric form substantially similar to that of the outer surface of said inner portion, said two surfaces being disposed in spaced relationship with regard to one another, said inner and outer sheathing portions each being pierced by at least one respective aperture, said aperture in said outer portion is displaced with respect to said aperture in said inner portion such that said apertures are disposed in a non-overlapping relationship to provide a tortuous air path of substantially nonlinear characteristics through said ionization chamber thereby greatly reducing undesirable wind effects on the ionization current in said ionization chamber.