Electrophorus

In 1775, Alessandro Volta [p110] described a device that he named "Elettroforo Perpetuo". The idea was already known from experiments of Wilcke, in Sweden, some years before (1762), but Volta popularized the instrument in the most practical form. The figure above is from the original paper, probably redrawn, and shows demonstration devices for the electrophorus built by Volta. The basic device consists of a plate C-C (fig. 1), made of metal or wood covered with metal foil, with rounded edges, called the "shield", connected to an insulating handle E, originally made of glass varnished with shellac varnish. Below there is a metallic "plate" A-A, covered with a layer of insulating resin B, making the "cake". The plate is actually not required, if the resin block is placed over a partially conducting surface, as a wooden table, and the operator is in contact with the ground. The plate is required in the experiments in the two last pictures, however. A modern version can use a plastic rod as the handle for the shield and a plastic plate for the cake. See the pictures below.

If the shield is placed in communication with the prime conductor of an electrostatic machine (fig. 1), with the plate grounded, the capacitor formed by the shield and the plate, with the resin as dielectric, is strongly charged. If then the shield is touched, with the plate being touched at the same time (or if the plate and the operator are grounded), the capacitor is discharged by a strong spark, but a certain amount of charge, that moved to the surface of the resin, is not removed. By rising the shield it's noted that it is now charged with a polarity opposite to the polarity at the resin surface, also contrary to the polarity of the electrostatic machine used to charge the device. A similar effect can be obtained by charging the resin B by frictioning it with a dry hand, with a flanel, etc. (fig.5), placing then the shield over it and touching the shield and the plate, which is the most usual form of charging the electrophorus. The resin can continue with an useful charge for hours, even for several days, if the humidity of the air is low. It's interesting to note that the charging method using an electrostatic machine, although clearly mentioned by Volta (and that works very well, see the video below), is practically not mentioned in the posterior literature about the electrophorus.

If the shield, after touched, is rised from the resin, its electrical potential is greatly increased. Sparks can be drawn from it, for example to charge a Leyden jar (fig. 3), that after a repetition for several cycles of the procedure of placing the shield over the cake, touching it and the plate, and rising it to touch the internal terminal of the jar, becomes strongly charged. The discharge of the jar generates a spark more intense that the electrophorus (fig. 3), due to the accumulated charge, but with the same maximum voltage.

An electrophorus can be used to charge another with opposite polarity (fig. 2). The electrophorus at the right acts as the Leyden jar in fig. 3, and the electrophorus at left acts as the electrostatic machine in fig. 1. The wire K is just for convenience. The shield of the receiving electrophorus can be directly touched with the shield of the generating one after it is lifted from its cake. When the right electrophorus is well charged, after several cycles, it can be used to reinforce the charge in the first, with both exchanging roles. Thus, two electrophorus form an influence electrostatic machine, and become both more an more charged as the clycles are repeated. This form of charging only works reliably if the first electrophorus is highly charged, otherwise very little charge moves to the resin surface of the second. A better charging method, described by Lichtenberg, is to roll the border of the shield of the first electrophorus over the cake of the second (see a video below). Another form, attributed to Klinkoch, is to simply move the same shield between two cakes alternately, touching it at each time (I could not make this work in this way, but could if the shield border is rolled on the cakes before being placed over them and touched).

A Leyden jar can be used to reinforce the electrophorus charge (fig. 4). It is first charged as in fig. 3, and then placed over the cake, held by the base. Then, the jar is left there and, held by the internal terminal, moved over the cake surface. The procedure causes the partial discharge of the jar, with charge that was at its external armature being transferred to the cake surface. It's also possible to leave the shield over the cake when the jar is placed over the assembly, what is analogous to the procedure in fig.2, but using the jar instead of the electrophorus at the left. It's then enough to touch the button of the jar to cause the discharge, but this procedure requires more charge in the jar to be effective.See the comments by Ingenhousz about these reinforcement methods here (Philosophical Transactions, Vol. 68, p. 1028, 1778).

In fig. 7, the lower plate is mounted over an insulating base, formed by a varnished glass rod E and a wooden base P-P. The base, in the original paper, is a box where the other devices can be stored (fig. 6), and that can also be insulated by insulating feet and used as an insulated platform, for example for the electrization of people. Points N are mounted in holes in the shield and in the plate, a sphere Q is mounted on the plate, and grounded points M are positioned as shown. The electrophorus is then charged and the shield is rised to the position shown. By observing the assembly in the dark, luminous discharges (corona) are observed at the points. In the picture, the shield is positive and the plate negative. Positive corona (a plume) is observed in the point connected to the shield and in the grounded point close to the sphere in the plate, and negative corona (a luminous point) is observed in the two other points. With the electrophorus charged with opposite polarity, the corona appearances are inverted. This experiment requires a large electrophorus to be clearly observable.

In fig. 8, the assembly is mounted upside-down, and a Leyden jar, with the external armature grounded and the internal terminal placed close to the ball connected to the shield, is charged by sparks when the cake is rised after the charge of the electrophorus. A spark can also be taken from the rised plate.

Electrophorus and Henley's electrometer.

The picture above shows a small electrophorus. The shield was made with an MDF plate with rounded edges covered with adhesive aluminum tape. A brass piece screwed on it fixes an acrylic rod used as handle. The cake is an acrylic round plate. There is no metal plate under it in this case. The quadrant electrometer of Henley's type (1772) serves to measure the voltage in the shield.

Videos: Charging the electrophorus with an electrostatic machine. This method is more efficient than to try to electrize the cake by friction, in a place not very dry. Charging an electrophorus with another, by the method described by Lichtenberg (1780). Works better than Volta's method.

A large version of the electrophorus was built by Lichtenberg in the XVIII century. The cake had 6' of diameter, the shield 5', and it could produce 14" sparks. Another large electrophorus was built by Kleindworth for the University of Gottingen. The cake was 2.25 meters in diameter and the shield had 2 meters.
A large electrophorus was recently built (2012) at the Fondazione Scienza e Tecnica, em Florence, Italy, similar to the one used by Lichtenberg. It is used for didactic demonstrations (photo sent by Paolo Brenni).

Volta's Electrophorus