Designed by Hamilton, the girandola is also known as an electric reel or windlass; it has five coplanar arms of thin brass wire attached radially to a hub, it too made of brass. The wires end in a very sharp point and are bent, almost at right angles, all in the same direction, on the horizontal plane to which they belong.
The support consists of a round brass base (diameter 10.7 cm), weighted down by a cast iron bottom and a glass insulating rod, at the top of which there is a thin steel pointed pin. The girandola is free to rotate on the steel pin, on which it is placed by means of the hub; the overall height is 20 cm.
Its use is to illustrate the effects of ionization of the air produced by strong electric fields that are created near the ends of charged conductors.
By connecting the brass support to an electrostatic machine, the girandola is charged and the charge collects at the extremities; the electric field generated in the surrounding area is thus quite intense and, if it surpasses the dielectric rigidity of the air, it ionizes it. The ions charged with the sign opposite to that of the extremities and very close to them are attracted and, on falling onto them, partially neutralize the charge.
The charge at the extremities is continually regenerated while around them charge clouds of the same sign are created; the reciprocal repulsive action causes a violent movement of the air in the direction indicated by the extremities (so-called electric wind) and a rapid rotation of the girandola in the opposite direction.
Daguin (1878), T. III, p. 157
Ganot (1864), p. 540
Privat Deschanel (1869), p. 578