to the length of time required for one complete sparking cycle i e the time for sparking to take place and then for implosion of the gas bubble and removal of debris before the next sparking event the impact of vibration becomes very significant for high levels of duty cycle but is negligible at low levels of duty cycle the hsedm apparatus described with reference to fig5 a and b has a linear induction servomotor which both displaces the electrodes to maintain the spark gap and produces the vibratory movement of the electrodes however other configurations are possible e g in which the displacement and vibration functions are driven by different parts of the apparatus for example fig1 a and b shows schematically respectively front and side views of an hsedm apparatus in which a lead screw servomotor drives the electrode displacement and separate piezo electric or pneumatic linear actuators drive the electrode vibration the servomotor 301 has a coupling 302 to a lead screw 303 that turns the servo rotation into linear motion of a head carriage 304 an electrical connector 321 and a pneumatic chuck 305 are provided on the head carriage 304 the electrical connector is connected to an electrical power supply omitted in fig1 a and b and transmits power across a mating connector 320 to tubular electrodes 311 mounted to a tool holder 308 the pneumatic chuck holds the tool holder to the head carriage under an electric signal command the tool holder 308 has an electrode cartridge 307 a noseguide assembly 314 carrying a static noseguide 313 is coupled to.