  High-resolution H$\alpha$ observations indicate that filaments consist of an
assembly of thin threads. In quiescent filaments, the threads are generally
short, whereas in active region filaments, the threads are generally long. In
order to explain these observational features, we performed one-dimensional
radiative hydrodynamic simulations of filament formation along a dipped
magnetic flux tube in the framework of the chromospheric evaporation-coronal
condensation model. The geometry of a dipped magnetic flux tube is
characterized by three parameters, i.e., the depth ($D$), the half-width ($w$),
and the altitude ($h$) of the magnetic dip. The parameter survey in the
numerical simulations shows that allowing the filament thread to grow in 5
days, the maximum length ($L_{th}$) of the filament thread increases linearly
with $w$, and decreases linearly with $D$ and $h$. The dependence is fitted
into a linear function $L_{th}=0.84w-0.88D-2.78h+17.31$ Mm. Such a relation can
qualitatively explain why quiescent filaments have shorter threads and active
region filaments have longer threads.
