研究两种平衡电路，它可以统一解释电位计、电桥和运放电路等各种零电路. 用于运放以后，平衡电路可以克服经典应用时的局限性，特别是它能使平衡电路自动地、瞬时地达到平衡，并且能处理从直流扩展到包括低频、高频的交流信号.

Two kinds of balance circuits are studied. They can explain various null circuits from a uniform view， such as potentiometer，Wheatstone bridge and operational amplifier. The usage on operational amplifier can overcome the limitation of classic amplification of balance circuit. Especially，it makes the balance circuit reaching equilibrium automotccally and immediately，and can treat the signal form DC to AC，including LF and HF.