If the normal functions of the grid and the plate of a conventional vacuum tube are interchanged, relatively large negative potentials applied to the plate can be measured as a function of the current drawn by one of the grids. By combining the characteristics of a tetrode operated in the inverted manner with a circuit employing a large amount of grid‐cathode degeneration, an instrument possessing an exceptionally high degree of stability has been constructed which is capable of measuring negative potentials from zero to 30 kilovolts. The indication is nearly linear with voltage and the input impedance is of the order of 1013ohms.