= Measurement method
A \b[measurement method] describes the general logical arrangement of operations used to make a measurement. A <measurement procedure> supplies the detailed instructions for implementing that arrangement with a particular <measuring system>. The VIM lists <substitution measurement methods>, <null measurement methods>, differential methods, and direct or indirect methods among the possible descriptions.<International vocabulary of metrology by JCGM>[VIM3entry2.5, definition, note and annotation dated11June2015]{ref}
For an original <substitution measurement method>, drive an unknown <resistor> with a fixed <electric current> $I$ and record a <voltmeter> indication. Replace it by an adjustable reference resistor without changing $I$ or the other relevant conditions, and adjust the reference until the indication agrees. In an ideal <measurement model>,
$$
v_x=IR_x+v_0,\qquad v_s=IR_s+v_0,
$$
so matching $v_x=v_s$ implies $R_x=R_s$. The common additive readout offset $v_0$ cancels. This cancellation depends on the current, connections and offset remaining common; the name of the method alone does not guarantee those conditions.
A <null measurement method> instead adjusts a reference to make a difference signal vanish, as in a balanced <Wheatstone bridge>. A resistance inferred from separately measured <electric potential difference> and <electric current> uses the relation $R=V/I$. These are descriptions of how the information is obtained; the operational record must still specify apparatus, settings, timing, reference values and the <measurement uncertainty>. The <measurement principle> concerns the physical phenomenon used, while the <measurement method> concerns the organization of the measurement. Different methods can exploit the same physical principle.
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