Ionization smoke detector

An ionization smoke detector comprising a first electrode having a head at the end thereof and a central axis, a source of radiation mounted on said head at the end of said first electrode, a metallic cup shaped member having a central axis aligned with and coaxial with said central axis of said first electrode and positioned with the interior thereof facing and being spaced from said first electrode, said cup shaped member defining a second electrode and having passage means through the walls thereof for admitting ambient air into the space defined within said cup shaped member and between said cup shaped member and said first electrode, a third electrode spaced axially outwardly from said head of said first electrode and laterally to one side of, or radially of, said central axis of said head of said first electrode, said first, second and third electrodes being adapted for connection to a smoke detector circuit and said radioactive source providing a predetermined ion current flow between said first electrode and said second electrode to establish a predetermined voltage potential at said third electrode, and the voltage potential at said third electrode being changed by the presence of smoke in said space with such change in potential being sensed by the smoke detector circuit for causing an alarm to sound.