Abstract
Modifications are described to an Edwards IBMA 1 ion beam thinning machine which permit the earth current from the specimen holder to be monitored continuously. The earth current, which gives an indication of the ion beam current, is used to trip out the HT supply to the ion guns and the timer whenever the ion current drifts outside a preselected range. These modifications have been carried out on two ion beam thinning units, which operate independently from the same high voltage power supply. Trials with the modified machine have shown that the thinning rates of aluminium, copper and glass specimens all increase when the specimen holder, which is normally insulated, is earthed through the current monitor.