Astatic galvanometer, 19th century.
© Royal Institution / Science & Society Picture Library
The accuracy of early galvanometers was limited by the fact that when electric current was runing, the instrument's needle was affected by two magnetic fields; the one produced by the instrument's electromagnet, and that of the Earth itself. The Italian physicist Leopoldo Nobili (1784-1835) solved this problem in 1825 by inventing the astatic galvanometer, which, by using a mobile magnet, cancelled out the effect of the Earth's magnetic field. This greatly increased the sensitivity and precision of the galvanometer to such an extent that, in 1827, Nobili was able to measure the electric current in a frog.