A perturbation method is used for the experimental study of the effects of end holes and a glass tube on the electric field configuration in a cylindrical TM010 microwave cavity at a wavelength of 3 cm. The radius of the cavity, the radius of the end holes in each end plate and the thickness of the glass tube are comparable. The results show that the electric field intensity and configuration in the plane of the end holes (i.e. in the interacting space of a plasma with the cavity) are highly perturbed. The effect of the glass tube is to make the electric field in the radial direction more homogeneous, and to increase the interacting space in an axial direction. The value of the square of the electric field intensity E2 at the centre of the cavity without the glass tube is nearly twice as large as the value found with the glass tube. The measured values of E2 are used to calculate the error due to the glass tube in the electron density measurement. The error is found to be 29%.