The paper describes a digital voltmeter which measures a voltage up to 10.23mV and displays the result as a 10-digit binary number of 10 ÿV steps. A visual output is provided, but the main purpose of the instrument is to feed the information to a tape punch which prepares an input tape for a digital computer. The instrument provides a direct link between an experiment or equipment in which voltages are required to be measured and a digital computer. The voltmeter employs germanium transistors, semiconductor diodes and relays; no thermionic valves are used. The method of measurement is to set up a potentiometer to produce a feedback voltage equal to the input voltage. The potentiometer consists of a number of substantially constant currents, proportional to the digit they represent, which are switched either to earth or through a common resistor. The potentiometer is set by an automatic trialand-error method starting with the most significant digit. As each digit is tried, the input and feedback voltages are compared and the sign of the difference between them determines whether that digit should be `1ÿ or `0ÿ. Since the display register of the instrument has 10 binary digits, this requires 10 operations and takes about 1 sec.
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