A wire of length $L$ and $3$ identical cells, of negligible internal resistance, are connected in series. Due to the current, the temperature of the wire is raised by $\Delta T$ in a time $t$. A number of $N$ similar cells are now connected in series with a wire of the same material and area of cross-section but of length $2L$. The temperature of the wire is raised by the same amount $\Delta T$ in the same time. Find the value of $N$.