On the wooden base stands a lathe-turned column on which is fixed a curved brass wire with a hook at the end. A thin silk thread attached to the hook suspends a magnetic needle at the centre of a rectangular copper coil. The ends of the conductor are joined to two clamps connected to the terminals of a battery.

The instrument is positioned so that the plane of the coil and the needle corresponds to the plane of the magnetic meridian; when the circuit is connected to a battery, the magnetic pointer changes direction due to the effect of the magnetic field produced by the current.

In instruments that measure current on the basis of this principle, the pointer moves over a graduated scale, so that current intensity can be measured according to the needle’s angle of deflection.

Device on display.