any electrical discharge machining of the electrodes the electrical discharges which in reality are microdischarges appear across the resistor r in the form of voltage pulses u of very short duration it is evident that in view of the capacitance between the surfaces of the electrodes 1 and 2 the voltage between the electrodes increases progressively after each microdischarge because the capacitor formed by the electrodes must be charged through the resistor r the time interval between two consecutive microdischarges therefore increases as a function of the triggering delay such that the number of microdischarges per unit of time provides a means for measuring the average triggering delay of the microdischarges the number of microdischarges is calculated by the circuit 5 fig2 which comprises a counter 6 having an input connected to the common terminal of the dc power supply 4 and of the resistor r the number of discharges counted by the counter 6 is stored in the memory 7 which in turn supplies the count to a visual display 8 a constant frequency oscillator or clock 9 is connected to the counter 6 and to the storage memory 7 such as to provide predetermined constant time intervals on one hand to store the number of microdischarges during such a pre determined time interval and on the other hand to re set the counter 6 to zero the electrodes illustrated at fig2 could have a different shape for example they could be conical or flat such as to permit adjusting the electrical discharge gap as a function of the solid particle concentration level a similar method.