Digital voltmeter

This unique instrument is a electro-mechanical vacuum-tube digital multimeter. It operates on a bridge principle. A servo circuit detects the output voltage of the bridge, which consists of the input voltage and a variable resistor (potentiometer). The potentiometer is mechanically linked to a motor and the dial. The servo circuit would switch on the motor to move the potentiometer (and the dial) until the bridge circuit would be balanced. This device was manufactured by little-known Aritma company in Prague. It is probably a prototype which was never mass produced.