generator 50 essentially determines the spark duration in the sample material 150 when relay 12 switches from position a to position b in response to a firing signal via line 16 the capacitor 15 begins discharging through the applying means 20 and the voltage dividing network 40 at this time the voltage dividing resistor network 40 instantaneously charges to the voltage level of the capacitor 15 sending a signal to the pulse delay generator 50 through line 45 this signal triggers the pulse delay generator 50 to begin timing delay after the preset delay time of the generator 50 a signal is sent along line 53 which triggers short circuiting means one preferred embodiment of the spark terminating circuit comprises a high voltage pulse generator 55 connected to a spark gap device 59 by line 57 the spark gap device 59 is connected in parallel with the capacitor 15 and spark igniting circuit 20 such that a short circuit is created across the capacitor 15 and spark igniting circuit 20 where a spark is transmitted across the spark gap device 59 upon receiving a triggering pulse from the delay pulse generator 50 the high voltage pulse generator 55 of the type available off the shelf and known to those skilled in the art sends a high voltage pulse along line 57 to spark gap device 59 this pulse is of sufficient electrical potential to create a spark across the gap of the spark gap device 59 the spark across the spark gap device 59 creates a low resistive path as seen by the discharging capacitor 15 which effectively short.