operational amplifiers op 7 and op 39 7 set sensitivity of the strain gauges g 1 and g 2 be nearly equal adjustment value when the tension balance between the upper thread and the lower thread is proper the operational amplifier op 8 is a first comparator which receives the output of the operational amplifier op 7 at the inverting input terminal of operational amplifier op 8 via a resistance r 17 and receives the output of the operational amplifier op 39 7 at its non inverting input terminal of operational amplifier op 8 a resistance r 18 zener diode zd and resistance r 19 r 20 is a voltage divided resistance and r 21 is a feedback resistance and the operational amplifier op 8 inverts and amplifies the output of the operational amplifier op 7 tr 1 is a first transistor which receives at its base the output of the operational amplifier op 8 via resistance r 22 and the collector of first transistor tr 1 is connected with a cathode of a luminous diode led 1 for indicating excessive lower thread tension which receives the power source v r 23 is a base emitter resistance and r 24 is a current limiting resistance the operational amplifier op 39 8 is a second.