Equivalent circuit of a 4 wire measurement system. On the left is a rectangular block labeled Voltmeter. To its right is a circle with an arrow inside indicating an ideal current source. To the right of the current source is a resistor labeled unknown resistance. They are connected together as follows. The top of the current source connects to one end of a resistor and the other end connects to the top end of the unknown. The bottom of the current source connects to one end of a resistor and the other end connects to the bottom end of the unknown. The top terminal of the voltmeter connects to a resistor and the other end connects to the top end of the unknown. The bottom terminal of the voltmeter connects to a resistor and the other end of this resistor connects to the bottom end of the unknown. All four of these resistors are labeled parasitic resistances.
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