[Hallicrafters] Measurement of Low Resistance

[Rich Oliver]

> The problem with ESR meters is that there is no way to compensate for
> the resistance of the meter probes in the circuit to be measured.
> Without knowing that you have good zero resistance contacts, the meter
> reading is suspect.
> 
> One way to check the resistance between antenna elements is to hook up
> a power supply and pass a known current through the junction. Measure
> the voltage drop between the tubing sections and compute the resistance.
> In this case the resistance of the power supply connections is not
> relevant as long as they allow sufficient current to flow. Using a
> high Z meter to measure the voltage insures that contact resistance
> of the probes is not significant. This is usually referred to as the
> 4 terminal method.
> 
> I'm sure you could come up with some variation that would allow the
> measurement of small resistances on a chassis.
> 
> 73, Roger

What Roger is describing is a Kelvin resistance measurement setup.  We 
had a commercial setup of this type back when I worked at Centralab in 
the distant past.  The most distinctive feature was a pair of split 
clips that acted as probes for the unknown resistance.  The two jaws of 
each clip are electrically isolated from one another.  One jaw in each 
clip is connected to the output of a current source, the other is 
connected to a voltmeter.  Resistance is simply V/I.  Using separate 
force and sense circuits effectively removes contact resistance from the 
equation, thus allowing accurate measurement of very low resistances.

Cheers, Rich, KC9GQ