The entire system is closed inside a cylindrical brass case (diameter 11.5 cm, height 8 cm), on a brass tripod with levelling screws: itsoverall height is 52.5 cm. The front of the case is polished glass, while the back is brass and has at its centre a small round glass window; on the sides of the case are the two terminals for connection. A large, slightly curved magneto can be moved on a vertical metal rod (height 26 cm) in the upper part of the case and turned, together with the rod by means of a screw for the purpose. In this way the disturbance of the earth’s magnetic field can be compensated for and the system made astatic. The movements of the compensating magneto also allow variation of the sensitivity of the galvanometer which is greatest when the action of the earth’s magnetic field is perfectly compensated for.

A beam of light from a fixed source and reflected by the mirror onto an external scale acts as the index. The high degree of sensitivity of this galvanometer is due to three important elements in its construction: The extreme lightness of the mobile element, the weakening of the disturbance caused by the earth’s magnetic field by the compensating magneto and the system of reading by means of a mirror.