The ion current from a Zinn‐type ion source has been determined for various values of three parameters of the source. These parameters are the diameters of the probe and plate holes and the spacing of the nose and plate. The ion current from the source was found to increase linearly with the diameter of the plate hole up to a certain value while above this value there was little further increase. The ion current likewise increased with probe hole diameter, but the increased gas flow prevented extensive increases in this parameter. A smaller gap between nose and plate gave the maximum ion current for a smaller current to the filament and therefore at a smaller power dissipation in the arc. Gas consumption was 10 cc per hour.