A voltage measuring device is described which is suitable for application to encapsulated, high-voltage switching installations which include an inner conductor surrounded by a grounded metallic outer tube. The device utilizes the fact that an electric field exists between the inner conductor and metallic outer tube. It includes means for widening a portion of the outer tube to thereby form a space for locating certain portions of the measuring device. The device itself includes an electro-optical component, such as a Kerr or Pockels cell which is positioned in the space defined by the widening means, at a distance from the inner conductor which is approximately equal to the distance that the outer tube is located from said inner conductor. A metal layer is positioned on that side of the electro-optical component which is away from the inner conductor. The metal layer is electrically connected to the grounded, metallic outer tube. The presence of the layer insures that the lines of the electric field between the inner conductor and the outer tube are directed through the electrooptical component, perpendicular to the metal layer. A source of polarized light is directed towards said electro-optical component. As the light passes through the cell the plane of polarization of the light is rotated as a function of the electric field intensity. Thus the exiting light becomes a measure of the voltage between the inner conductor and the outer tube. The shifted, polarized light exiting from the electro-optical component is then processed by an optical evaluation device which produces an electrical output proportional to the amount of shift of the plane of polarization of the light.