  We reveal and explain a scaling behavior of the thermopower $S/T$ exhibiting
by the archetypical heavy-fermion (HF) metal $\rm{YbRh_2Si_2}$ under the
application of magnetic field $B$ at temperatures $T$. We show that the same
scaling is demonstrated by such different HF compounds as $\beta$-${\rm
YbAlB_4}$ and the strongly correlated layered cobalt oxide $\rm
[BiBa_{0.66}K_{0.36}O_{2}]CoO_{2}$. Using $\rm{YbRh_2Si_2}$ as an example, we
demonstrate that the scaling behavior of $S/T$ is violated at the
antiferromagnetic phase transition, while both the residual resistivity
$\rho_0$ and the density of states $N$ experience jumps at the phase
transition, making the thermopower experience two jumps and change its sign.
Our elucidation is based on flattening of the single-particle spectrum that
profoundly affects $\rho_0$ and $N$. To depict the main features of the $S/T$
behavior, we construct the $T-B$ schematic phase diagram of $\rm{YbRh_2Si_2}$.
Our calculated $S/T$ for the HF compounds are in good agreement with
experimental facts and support our observations.
