English: A precision Ampere balance at the US National Bureau of Standards (now NIST that served as a standard for comparing electric current for the United States in 1927. This specific type was called a Rosa-Dorsey current balance. It consisted of a sensitive laboratory balance (top) with one of its pans replaced by a coil of wire (visible under balance), with a second stationary coil of wire below it. An unknown current is passed through the coils, causing the upper coil to be attracted to the lower coil, pulling the right arm of the balance down. Standard weights are put on the lefthand pan until the arms balance. The magnitude of the unknown current can be calculated from the weights and the dimensions of the coils. The accuracy of the Ampere balance was limited because it was dependent on the shape and rigidity of the wire coils. It was replaced as a standard of electric current in the 1960s by atomic standards, such as Josephson junctions.