servo control circuits particularly suited for electric discharge machining apparatus to provide the input to the servomechanism which actuates the tool feed during machining . these circuits respond to a setting of desired gap emf and an input of actual gap emf to control tool movement so as to equalize these inputs . they respond to abnormally low gap emf during the machining &# 34 ; on &# 34 ; pulse to retract the tool to allow the gap to clear of debris . the gap emf during the &# 34 ; on &# 34 ; period of the pulse cycle is stored in a small capacitor coupled to a high - impedance input to power circuits of the servo control , which capacitor normally retains most of its charge during the &# 34 ; off &# 34 ; time of the cycle . logic circuits responding to the pulse generator and the gap emf discharge this capacitor when abnormally low gap emf occurs during cycle &# 34 ; on &# 34 ; time . the transistorized circuit directly supplying the servo valve operating coil provides for control by inexpensive low power elements of small size and low energy dissipation .