This is a particularly sensitive astaticgalvanometer devised by the English physicist Sir William Thomson (1842-1907) (Lord Kelvin) who had a prototype built by the Elliot Brothers in London.One that is identical to the one in Cagliari, built by the same Elliot Brothers, is to be found in the Teyler Museum in the Netherlands; the only difference is that the one in Cagliari bears the number 862 while the other is number 539.

In the centre of a vertical coil a very light mobile element (its weight is no more than a tenth of a gram), composed of five small, thin magnetic needles attached horizontally to the non-reflecting face of a circular mirror (diameter 8 mm), one along the diameter and the others, two by two placed symmetrically with respect to the first one; the central needle is the same length as the diameter and the others have the same length as the corresponding chord. The mirror is attached to a very lightweight rod with a very light lamina of mica attached to the lower end. The rod is suspended on a thin silk thread from a knurled screw which allows its height to be adjusted.

The coil is composed of two coils in series that can be set at different dsistances from each other to allow intreoduction of the mirror; at the level of the latter the coil has an aperture in the form of a truncated cone having more or less the same dimension as the mirror in the innermost part and gradually widens towards the outside. ...continues