Electrostatic machines, or generators, are devices capable of separating with continuity, at the expense of mechanical energy, charges of the two signs, delivering them to two metal pieces (poles), usually spherical in shape, between which there is consequently established a larger or smaller difference in potential.
The electrostatic machine invented by the English engineer James Wimshurst (1832-1903) in 1883, thanks to the use of a brush device for the collection of separate charges and two Leyden jars for their storage, is capable of storing large charges and establishing between its poles much greater differences in potential compared to previous machines.
It consists of two identical glass or hard rubber disks which, by means of a crank, are made to rotate in opposite directions on a common horizontal axis.
On the sides of the disks not facing each other, near their edges, there are sectors of tin foil arranged diagonally two by two.
During rotation, each pair of sectors is for an instant connected by a diametrical conductor having at its ends brush contacts made of soft copper wire. Two U-shaped brushes placed at the extremities of the horizontal diameter touch both disks and are connected to the poles of the machine and to the inner armature of a two Leyden jars, the external armatures of which are connected.
When a charged sector passes in the vicinity of one of the brushes, this and the inner armature of the Leyden jar to which it is connected are charged by induction; The charge of opposite sign which is manifested in the brush neutralizes that of the sector (power of the points) and the charge of the same sign is stored in the Leyden jar.
As soon as the disks begin to rotate, especially if they are made of hard rubber, they are charged by the brushes of the diametrical conductors and the charge thus accumulated from one of the sectors or a residual charge, even when quite small, is sufficient to set off a rapid process of multiplication by induction (self-excitation induction machine).
In fact, when one sector of one of the disks that is initially charged in the rotation is in correspondence to the diametrical conductor of the other disk, the two sectors that connect the latter are charged by induction and, on separating from the diametrical conductor, they remain charged.
Since the two disks rotate in opposite directions, the influencing sector and the sector farthest away, which are on opposite sides with respect to the horizontal diameter and are charged with the same sign, go towards the same brush; while the one closest to the influencing sector, which is on the same side as the horizontal diameter and is charged with the opposite sign, goes towards the other brush.
Each diametrical conductor is placed at approximately 60 degrees from the horizontal diameter in the direction of rotation of the corresponding disk and therefore the other diametrical conductor is at approximately 60 degrees from the former, in the same direction.
The result is that before arriving in proximity to the brush, each of the two diametrical sectors charged by induction pass in correspondence to the diametrical conductor of the other disk; the two sectors connected by this are charged by induction, the process repeats itself and continues for as long as the machine is working.
All things considered, since the two disks rotate in opposite directions, at the generic instant of the functioning of the machine, the sectors in the upper half of one of the disks and those in the lower half of the other disk, which are charged with the same sign, go towards one of the brushes and those in the other two halves go towards the other brush.
The specimen presented here has been modified with the substitution of the original hard rubber disks with Plexiglas disks of the same size (diameter 3.5 cm), with the same number of sectors (16) and by inserting two screws at the extremities of the diametrical conductors to attach and regulate the brushes.
The inventory number that follows, bearing the same date of registration, has been attributed to a spare hard rubber disk.
Battelli-Cardani (1925), Vol. IV, P. I, p. 297
Ragozzino-Schettino (1985), p. 25
Roiti (1913), p. 100
Turner (1983), p. 192