Keithley Instruments

Keithley Instruments is an instrument manufacturer associated with precision electrical measurement. Its Low Level Measurements Handbook, Seventh Edition by Keithley Instruments explains how current sources, voltmeters and connection arrangements determine what a low-level reading means, with concrete treatments of lead resistance and thermoelectric offset.^[ref]

A useful example is the handbook's comparison of two-wire resistance measurement with four-terminal sensing. In two-wire measurement, current and voltage readout share the leads, so their resistance contributes to the measured drop. Separate voltage-sensing connections can avoid including that lead drop when sensing current is negligible.^[ref]

For an original ideal calculation, a $1\Omega$ resistor has two current leads of $0.1\Omega$ each. At $10\mathrm{mA}$, the total drop is $12\mathrm{mV}$, giving $1.2\Omega$ if all of it is attributed to the specimen. A separate voltmeter connected directly across the specimen, with negligible sensing current, instead measures $10\mathrm{mV}$ and gives $1\Omega$.

That result follows from the connection arrangement, rather than from the manufacturer's name. Finite input impedance, contact placement, thermoelectric offset and source uncertainty can still matter. The manufacturer's technical handbook is useful as an apparatus-level account of such limits; its example methods do not establish the accuracy of an unspecified instrument or a particular laboratory setup.