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An automatic electronic visual-reaction-time meter has been developed which includes a random-pulse generator controlled by nuclear radiation for starting the subject's light pulse. With this instrument (using radioactive ion generators) several hundred subjects have been investigated in over 12,900 tests. An influence of light atmospheric ions on the human reaction time has been found for ion currents between 106 and 109 inhaled ions per second or ion densities of about 103 to 106 ions/cm3. Inhaling positive or negative ions may increase or decrease the reaction time of different people or even of the same person after several hours. In this respect the influence of ions resembles the effect of many drugs on the human system. The effect disappears when the subject is breathing through the nose instead of the mouth.