Three identical capacitors ${C_1},{C_2}$ and ${C_3}$ have a capacitance of $1.0\,\mu F$ each and they are unchanged initially. They are connected in a circuit as shown in the figure and ${C_1}$ is then filled completely with a dielectric material of relative permittivity ${\varepsilon _r}.$ The cell electromotive force $$\left( {emf} \right)\,\,{V_0} = 8V.$$ First the switch ${S_1}$ is closed while the switch ${S_2}$ is kept open. When the capacitor ${C_3}$ is fully charged, ${S_1}$ is opened and ${S_2}$ is closed simultaneously. When all the capacitors reach equilibrium, the charge on ${C_3}$ is found to be $5\,\mu C.$ The value of ${\varepsilon _r} = \underline{\hspace{2cm}}$.