The first voltmeter, called a galvanometer, was invented by Andre-Marie Ampere in 1824. It was the first instrument used to measure the electrical current in a conductor.
The early galvanometers were not very accurate or consistent. Modern voltmeters measure the potential difference between two points on an electrical circuit. The first vacuum tube voltmeter was invented in 1922 by E.B. Moulin at the University of Cambridge.
The first digital voltmeter was invented by Andrew Kay of Non-Linear Systems in 1954, leading to further developments, including the dual slope analog-to-digital conversion circuit. The use of digital technology has improved the accuracy of the voltmeter to within 1 percent.