Nobili's galvanometer is of the astatic, portable kind with a coil on the small wooden frame and a cylindrical glass case (diameter 8.5 cm, height 9.5 cm), on a circular base (diameter 18.5 cm, height 3.6 cm) of black lacquered wood with three levelling screws (overall height 17 cm). The terminals for connection are on the side of the base and a screw is provided for zeroing the instrument prior to use by rotating the coil.
The astatic element is composed of two identical magnetic needles fixed perpendicularly to a small brass rod and parallel to each other, but with the poles oriented in opposite directions; the device is suspended from a hook on a silk thread. The hook has an adjusting screw at the centre of the lid of the case.
One of the needles can rotate inside the coil as in ordinary galvanometers, and the other, above it, acts as the index for the graduated scale. The system with the double needle, besides eliminating the influence of the earth's magnetic field, makes the galvanometer more sensitive than one with a single needle; in fact, the current flowing through the coil acts in the same sense on the two needles, causing ampler rotations than would be caused if the galvanometer had only one needle.
The outside needle, which also acts as the index, is near a copper disk which, by electromagnetic induction, dampens the oscillations.