This paper presents the results of experimental studies on a number of different ‘‘current carrying wire’’ type first grid electron suppressors: or ‘‘inserts’’, as they shall be called throughout this paper. Extracted negative ion currents are compared for the different inserts over a range of field strengths. Currents are also compared to those extracted with no suppressor present. An attempt is made to relate the negative ion currents obtained for each insert, with field depth and uniformity. Results are presented on the extraction of negative ions with an insert operating some distance into the source. The insert is also tested with a dipole rather than tent field used to separate the driver and extraction regions of the source.