Bridge Circuit Construction:

Above Figure displays a basic bridge circuit that consists of three knew resistances, R₁, R₂, and R₃ (variable), an unknown variable resistor R_X (RTD), a source of voltage, and a sensitive ammeter.

Figure: Bridge Circuit

Resistors R₁ and R₂ are the ratio arms of the bridge.  That ratio the two variable resistances for current flow through the ammeter. R₃ is a variable resistor called as the standard arm that is adjusted to match the unknown resistor.  The sensing ammeter visually displays the current that is flowing through the bridge circuit.  Analysis of the circuit displays in which while R₃ is adjusted so that the ammeter reads zero current and the resistance of both arms of the bridge circuit is the same. The Given Equation 1-1 shows the relationship of the resistance between the two arms of the bridge.

R₁/R₃ = R₂/R_x                                                                                      (1-1)

Because the values of R₁, R₂, and R₃ are known values, the only unkown is R_x.  A value of R_x could  be  calulated  for  the  bridge  during  an  ammeter  zero  current  condition. Getting this resistance value provides a baseline point for calibration of the instrument attached to the bridge circuit.  The unknown resistance R_x, that is provided through Equation 1-2.

R_x = R₂R₃/R₁                                                                                                      (1-2)