Since the voltmeter internal resistance trends to infinite, the measuring current is very small, typically 10 to 100 pA. Thus, the voltage drop on the current sense wire is very low compared to the voltage drop on the subject component forced by the main current. This is the reason that the resistance of the current sense wire has no significant influence on the measuring result. Another advantage of using the four-wire method to measure resistance, is that it also helps to prevent the influence of any damaged wires or probes on the resistance measurement. Microhm's 4 terminal connection resistors include high precision power resistor MVR2321-4, MVR3825-4,MVR4618-4, shunt resistors and more.
While two-wire method are suitable for measuring resistance vaules greater than 10 ohms as the resistance value being measured is much higher than the typical resistance of the measuring wire.
Most digital multimeters will use a two-wire method to measure resistance. The disadvantage of this method is that not only the resistance of the component but also the total resistance of the circuit is measured. This method provides the total resistance of the measuring cables, connectors, fixtures and clips and is typically 5 to 100 milliohms, which is incorrectly added to the
measured result.
This method is also sensitive to contact probe failure or cable failure, and will introduce additional errors if those problems exist. The measurement of lower resistance values under 10 ohms using this method likely will be inaccurate, especially for metal strip current sense resistors with resistance ranges of 50 micro ohms to 100 milliohms.
