Current measurement on low resistance sources
One way to measure very low resistances is to pass a relatively large current through the device under test (DUT), with a Kelvin (4-wire) connection. Then a precision amplifier can be used to measure the voltage drop. 



For example, suppose you have a 1A current source and an amplifier with 10mV full scale input (and say a 3-1/2 digit ADC). Then you can measure with 5\$\mu\Omega\$ resolution. 

It may be better to use +/-1A alternating and make alternate +/- measurements in order to get rid of thermoelectric voltages, but that's a detail.