A device for measuring electric charges of the order of 1.5×10−11coulomb with a probable error of approximately one percent is described. The apparatus was designed to measure the ionization produced in air by roentgenographic apparatus. For that purpose the apparatus consists of two ionization chambers, an FP‐54 electrometer tube, and an associated shielded switch. The grid of the electrometer tube is disconnected during the x‐ray exposure. The shielded switch is designed to minimize drift in the plate current while the grid is floating and to minimize extraneous effects when the grid is connected successively to the two ionization chambers.