  The energy dissipation of quasiclassical homogeneous turbulence in superfluid
$^4$He (He II) is controlled by an effective kinematic viscosity $\nu'$, which
relates the energy decay rate $dE/dt$ to the density of quantized vortex lines
$L$ as $dE/dt=-{\nu'}({\kappa}L)^2$. The precise value of $\nu'$ is of
fundamental importance in developing our understanding of the dissipation
mechanism in He II, and it is also needed in many high Reynolds number
turbulence experiments and model testing that use He II as the working fluid.
However, a reliable determination of $\nu'$ requires the measurements of both
$E(t)$ and $L(t)$, which was never achieved. Here we discuss our study of the
quasiclassical turbulence that emerges in the decay of thermal counterflow in
He II at above 1 K. We were able to measure $E(t)$ using a recently developed
flow visualization technique and $L(t)$ via second sound attenuation. We report
the $\nu'$ values in a wide temperature range determined for the first time
from a comparison of the time evolution of $E(t)$ and $L(t)$.
