The plates of a parallel plate capacitor are charged to a potential difference of $117 V$ and then connected across a resistor. The potential difference across the capacitor decreases exponentially with respect to time. After $1 \mathrm{s}$ the potential difference between the plates is $39 \mathrm{V}$, then after $2 \mathrm{s}$ from the start, the potential difference ( in $\mathrm{V}$ ) between the plates is