Reason Why Constant Current Bridge Voltage is used for Civil Enginnering Measurement with Cable Extended

See Fig. 1 at the right and suppose that E is the voltage of the bridge excitation and E' is the voltage applied to the transducer in the constant voltage bridge system. Then,

E'＝E−2･I･r

where, "I・r" is the voltage decrease due to cable resistance and "2・I・r " is that due to the reciprocating cable resistance.

With the constant current system, the current, I, is constant at all times, and thus the bridge output receives no influence from r.

This means that extension cable resistance does not cause any output decrease. However, the input resistance, Rg, of the bridge affects the output. But a difference between the nominal bridge resistance of civil engineering transducer and its actual bridge resistance is extremely small, and thus it need not consider that the input resistance of the bridge affects the accuracy.