Measure resistance

For producing an accurate measurement of something in the 0.1 ohm range I suggest that a four-terminal system will be the simplest to keep operating with reasonable accuracy. One set of 2 leads would supply the current to flow through the thing being measured, and the second set of leads would then read the voltage developed by the current passing through the resistance. Using 100Ma, that is, 0.100 amps, the scale factor would be 100 millivolts per ohm Thus 0.1 ohm would give 10 millivolts, a value fairly easy to read on a reasonably priced digital voltmeter. Adding an instrument amplifier with a gain of ten or even 100, would allow high resolution readings without a large amount of expense. Fixed gain instrument amplifiers are available from several reputable makers for a quite reasonable price. Regulating the current can be done simply with an LM7805 regulator IC and a reasonably stable DC power supply.

Tghe one thing not mentioned so far is the accuracy and the resolution required for the application.