What is the 4-wire sense function of the Keithley 2400 Sourcemeter measuring?

[Cara]

Hello all,

I am attempting to use the Keithley 2400 Sourcemeter to measure the changing resistivity of a thin film, in the 4-wire sense mode.  My LabVIEW driver is working fine, but I am obtaining some odd measurements.  In the 2 wire sensing mode, the resistance was measured to be about 7.5 Ohms and increased based on our manipulations (as expected) while in the 4 wire sensing mode the resistance dropped to 2 Ohms and decreased, which seems inexplicable.  My understanding is that the 4 point probe is measuring the current sourced across one set distance and the voltage dropped across another.  I am not an expert in electronics.  I would greatly appreciated some clarification regarding what precisely this function is measuring that would changed the measured resistance so drastically.

Thank you in advance,

Cara

[AnalogKid2DigitalMan]

Cara:

Probably a question better suited to Keithley, not NI.

Have you verified your probe connections are good and tight. Also, you need to make sure you + (red probes) are together on one side of the device and the - (black probes) are together on the other side of the device undergoing measurement.

Have you tried the measurements manually without using and LabVIEW code?

Here's a link to the manual, Chapter 4 explains 2 and 4 wire resistance measurements.

http://www.keithley.com/support/data?asset=887

Hope this helps.