Abstract: Automatic high insulation switch with low consumption level and small overall dimensions. The switch is associated with an apparatus for measuring very low currents (10.sup.-7 to 10.sup.-14 amperes) comprising an amplifier controlled by an input field effect transistor. The invention aims to switch on the amplifier's very high value resistors with the aid of magnetically controlled, flexible reed interrupters or switches, actuated by a permanent magnet, which is itself controlled by a conventional servomechanism having a motor and a potentiometer controlled by a microprocessor. Other interrupters ensure the protection of the transistor. The invention is more particularly used for measuring currents supplied by ionization chambers.