  Using micromechanical force magnetometry, we have measured the magnetization
of the strong-leg spin-1/2 ladder compound (C$_7$H$_{10}$N)$_2$CuBr$_2$ at
temperatures down to 45 mK. Low-temperature magnetic susceptibility as a
function of field exhibits a maximum near the critical field H_c at which the
magnon gap vanishes, as expected for a gapped one-dimensional antiferromagnet.
Above H_c a clear minimum appears in the magnetization as a function of
temperature as predicted by theory. In this field region, the susceptibility in
conjunction with our specific heat data yields the Wilson ratio R_W. The result
supports the relation R_W=4K, where K is the Tomonaga-Luttinger-liquid
parameter.
