I need to measure the reistance of a welded joint.The value of the joint would be in the range of 10 ~ 14 milliohms.My present system has a Constant current source of 500 mAmps at 12 Volts.When I apply the current source to the measuring points the Voltage drop occurs.This millivolt is then fed to an amplifier.Can someone guide me to build the constant current source.Thanks in advance