= Measuring system
A \b[measuring system] combines one or more <measurement instruments> with the other devices needed to produce measured values for specified quantities over specified intervals. A system can also consist of a single instrument when it performs the required function on its own.<International vocabulary of metrology by JCGM>[VIM3entry3.2, definition and note; entry3.1, measuring instrument]{ref} The system boundary identifies which physical components and signal-processing operations contribute to a <measurement result>.
For an original electrical example, assemble a <current source>, a test <resistor>, two connecting leads, a <voltmeter> and the calculation used to infer <electrical resistance>. The intended <measurand> is the test resistor's resistance $R_x$ at stated conditions. In a chosen ideal model, the current is $I$, the leads contribute total resistance $R_L$, and the readout has additive <measurement zero offset> $V_0$:
$$
V_{\rm indicated}=I(R_x+R_L)+V_0,
\qquad
R_x=\frac{V_{\rm indicated}-V_0}{I}-R_L.
$$
Here the voltmeter measures an <electric potential difference> across the complete resistor-and-lead path, with negligible input-current loading.
\Image[/measuring-system-lead-resistance.png]
{title=An electrical measuring system includes the connecting leads and readout response}
{description=Own ideal circuit with stipulated1mA current,100ohm test resistor, two1ohm leads and1mV readout offset. The indicated103mV includes the102mV physical drop and1mV offset. Negligible voltmeter loading is assumed; these are teaching values rather than measured instrument performance.}
With $I=1\,\mathrm{mA}$, $R_L=2\,\Omega$, $V_0=1\,\mathrm{mV}$ and indicated potential difference $103\,\mathrm{mV}$, the model gives $R_x=100\,\Omega$. Dividing the indication by the current without either correction would instead give $103\,\Omega$. The numerical resolution of the voltmeter alone therefore does not describe the whole measuring system's result.
A <measuring chain> describes successive signal transformations within such an arrangement. Its physical response and subsequent calculation belong in the system description, together with <measurement calibration>, reference conditions and relevant <measurement uncertainty>. Changing a lead connection, source setting or correction can change the result while leaving the main readout instrument unchanged. The <measurement procedure> records how that particular system is operated.
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