We experimentally determine the conjecture that hydrogen-like atoms such as Rb and Cs may have large permanent electric dipole moments (EDMs). The saturated Rb vapour fills a cylindrical capacitor in the experiment.The influence of the vapour dielectric medium on capacitance is measured with a digital capacitance meter. Supposing that the measurement influence comes from the permanent EDMs of Rb atoms, from the experimental result we find that the EDM is large, i.e. dRb ≥ 8.6e × 10-9 cm.