Sagitta
= Sagitta
{title2=$s=R-\sqrt{R^2-L^2/4}$}
The maximum distance between a circular arc and its chord of length $L$. For radius $R$ and $L\ll R$, expansion gives $s\simeq L^2/(8R)$. In a uniform transverse <magnetic field>, a charged track has $R=p/(|q|B)$, so the <sagitta> measures its <momentum>.