In the 3 wire configuration 1 wire goes to one side of the resistor and the other two go to the other side. This allows the use of a bridge circuit to cancel the actual wire resistances. In the four wire configuration two wires go to each side of the resistor. Two are used to supply the excitation current and the other two to sense the voltage across the resistor. Since voltage measuring circuits can be very, very high impedance the resistance of the wires connected to the voltmeter is immaterial. Since excitation is by a precision current source the resistance of the excitation wires doesn't matter either.One wire goes to one side of the sensor and the others all go to the other side.
In the 3 wire configuration 1 wire goes to one side of the resistor and the other two go to the other side. This allows the use of a bridge circuit to cancel the actual wire resistances. In the four wire configuration two wires go to each side of the resistor. Two are used to supply the excitation current and the other two to sense the voltage across the resistor. Since voltage measuring circuits can be very, very high impedance the resistance of the wires connected to the voltmeter is immaterial. Since excitation is by a precision current source the resistance of the excitation wires doesn't matter either.
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