end to a positive input of the another differential amplifier 11 thus the first second and third resistors r 9 r 10 r 11 form a summing circuit for forming the sum v 6 v 7 wherein v 6 is the potential of the second connecting point 6 and v 7 is the potential of the third connecting point 7 a negative input of differential amplifier 11 is connected through a resistor r 12 to the first connecting point 5 between resistors r 7 and r 8 a feedback resistor r 13 is connected between an output and the negative input of the another differential amplifier 11 the realization of the method according to the present invention occurs through transducer and the voltage v s at an output a of the instrument 8 can be expressed according to the following formula v s v 6 u2212 v 5 v 7 u2212 v 5 k where k is a constant that represents the gain of the instrument 8 where v s has to be equal to zero or to be between a range of preset minimum and maximum values v 5 v 6 and v 7 are voltages at first second and third connecting points 5 6 7 respectively v s has to be equal to zero or lower than a predetermined reference value v ref if there arises a crack in.