Ionization of helium by potassium positive ions

A continuation of previous attempts to ionize gases by positive ions has been extended to helium using potassium positive ions as high as 750 volts accelerating potential. At pressures between 0.01 and 0.1 mm there is definite evidence of ionization of helium above 150 or 200 volts accelerating potential. The effect is much smaller than in the gases previously reported, but it may be distinguished from the secondary emission of electrons from the metal parts under action of the positive ions. Great difficulty was encountered in changes of intensity of ionization due to impurities in the helium. Some of the uncertainties in previous work have been removed by changing the relative positions of electrodes.