For years inexpensive Digital Volt Meters (DVM) have been available to the amateur radio world. Professional ones are also available, but their prices are sometimes prohibitive to the amateur. In the professional world, four lead ohm-meters are used for measuring resistance.
In a basic measuring set up with just two leads, the result is compromised by the resistance of the leads connecting the device under test (DUT), i.e. the resistor in this case to the DVM. It is particularly troublesome when trying to characterise low value resistors.
If the leads have say 0.05Ω total resistance, and the DUT is a 0.1Ω resistor, the measurement will be wrong by 50%, as the DVM will show 0.15Ω instead of 1Ω. The way around this is easy to achieve with modern op-amps, and is achieved by using 4-wire Kelvin leads, some circuitry, and a DVM.
The circuitry will need to provide a constant known current through the DUT from say a Jfet operating in constant current mode. This known current is connected such that it goes in series with the DUT. The other two leads of the probe are connected to the DVM set to read volts. Some DVM are four lead models and do all this automatically. These four-lead models then calculate and display the value in ohms. The simpler arrangement will require the operator to work out the ohms from the current and voltage values.
An example of a commercial lead is shown below, but are quite easy to assemble for the hobbyist.
73, Richard, G3ZIY
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