= Keithley Instruments
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\b[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>.<Low Level Measurements Handbook, Seventh Edition by Keithley Instruments>[Chapter3, Measurements from Low Resistance Sources; §§3.3.1–3.3.2]{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.<Low Level Measurements Handbook, Seventh Edition by Keithley Instruments>[§3.3.1, Figures3-16–3-17, two-wire and four-wire arrangements]{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.
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