  Response in electric potensial $U$ from a DC RC circuit in series has been
obtained by varying the capacitance of the capacitor $C$. Variation of the
capacitance begins at $t_0 > 0$. Alternating response is observed when the
periode of change of capacitance in the same order as $\tau = RC$, which is
held constant for $t < t_0$ and varied in triangle form for $t > t_0$.
