Measuring instrument and amplifier for measuring very small DC and AC voltages, e.g. thermovoltages, inductive voltages and photo-electric voltages. With LED display. The measuring input includes a filter that can be switched in for smoothing the signal or for setting an upper frequency limit. The signal is input via a BNC socket or 4 mm safety sockets. An additional DIN socket is provided for the connection of Hall sensors, e.g. the Axial/Tangential Magnetic Field Sensor or the Flexible Magnetic Field Sensor.