Ultra‐high vacuum pumps of the sputter‐ion pattern have been investigated from the standpoint of origin and destination of electrically charged particles constituting pump current. It has been found that secondary electron emission is responsible for a large portion of total current, that ion energies range from 0.50 to 0.80 of applied pump potential, and that needle crystals (whiskers) that grow progressively from pump electrodes cause pump current to depart from its linear dependence on pressure.