100 Ohm resistor being measured: Top circuit uses a coventional 2-wire sense that most of us use with a DMM or scope. It has a contact resistance on each "probe" (1 Ohm). Bottom circuit uses a 4-wire sense. As you can see, the 4-wire sense yields a more accurate measurement. This is due to how the contact resistance is connected to the forcing current and voltage sense in each circuit.
