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This is a Simple 1 Amp Current Injector Circuit Diagram. A voltage booster provides the current control circuit with a higher voltage, to insure the electronic circuit operates properly, even as the battery voltage drops. A single LED provides both power monitoring and a low battery indicator. The current source is activated when a pushbutton switch is pressed. With a good digital voltmeter, you can use this device to measure resistances down to less than one micro ohm. 

That is 0.000001 ohms. I often use it to measure contact resistance of a relay or the resistance of big chunk of heavy gauge wire or the resistance of a copper trace on a circuit board. You can also use it to measure the voltage drop of a power diode, when conducting one amp of current.