26 the output of the comparator 25 is connected to one of the inputs to the comparator 18 and to one of the inputs of the pulse generator 3 through a line 28 a function generator 27 connected to the pulse generator 3 via a line 29 is programmed according to coded data supplied by an encoding circuit 30 the different couples of electrode tool electrode workpiece combinations being coded according to material compositions and other technical considerations according to a first mode of operation the machining gap 5 is maintained at a constant value by the servo control 23 when the contamination of the machining fluid varies in the course of a machining operation this results in a variation of the average delay of triggering of the electrical discharges in conformity with what is represented on the diagram of fig1 as soon as the value of the average triggering delay does not coincide with the reference value set by the potentiometer 26 the comparator 25 modifies at its output the value of the reference signal adjusting the concentration of solid particles and the signal at the output of the comparator 18 controls the proportioning device 11 until the measured signal representative of the concentration is equal to the reference value simultaneously with adjusting the concentration of solid particles in the machining fluid the comparator 25 provides a signal on line 28 which is applied to the pulse generator 3 such as to vary the triggering voltage of the pulses and such as to accelerate the adjustment of the average triggering delay for example in the event that the average triggering.