 Gauge block wringing

Gauge block wringing joins suitably finished measuring faces by bringing them into intimate contact with a controlled sliding or twisting motion. The resulting adhesion lets gauge blocks form a composite length standard or contact an auxiliary plane for interferometry. The contact layer is part of the length convention: a calibrated block length includes one wringing-layer contribution.^[ref]

Adhesion is not explained adequately by saying that air was removed and atmospheric pressure alone holds the blocks together. NIST's handbook describes multiple surface- and fluid-dependent contributions, including surface tension, and warns that film thickness varies with surface condition and time. It does not supply a universal predictive film model.^[ref]

An original bookkeeping model explains the length convention. Let $g_i$ be a block's geometric face separation and let $w_i$ be the effective layer contribution in its calibration, so its defined length is $l_i=g_i+w_i$. If a stack of $n$ blocks is wrung to an auxiliary plane, it has $n-1$ internal contacts and one base contact. Its top height can be writtenThus compatible contacts can make defined lengths additive without treating every interface as an additional correction beyond the certificates. Differences between calibration and use remain a wringing-film uncertainty.

For three stipulated blocks each carrying an effective $10\mathrm{nm}$ calibration contribution, three equivalent contacts contribute the same $30\mathrm{nm}$ already counted in their defined lengths. If one actual contact contributes $25\mathrm{nm}$ instead, the height in this simplified model differs by $15\mathrm{nm}$. These chosen values illustrate the convention; they are not a measured stack or a universal film specification.

Experimentally, NIST describes viewing a block wrung to a transparent optical flat: a uniform contact appearance supports a satisfactory wring, while coloured or bright regions indicate poor contact. Rewringing and checking the optical result tests an interface contribution that merely adding nominal lengths would miss.^[ref] Cleanliness, surface damage and thermal expansion during handling still affect the comparison. A visibly adhering stack is not, by itself, proof of the required dimensional accuracy.