Electrophore

The generator.

In experimental physics, the electrophore is a capacitive static electricity generator made up of a simple plate capacitor, manually operated. It produces electrostatic charges through an electrostatic induction process.

The first electrophore was invented in 1762 by the Swedish physicist Johan Carl Wilcke (1732-1796). Then Italian scientist Alessandro Volta perfected and popularized this device in 1775, which has led to his being wrongly credited for inventing it. Volta was the one who coined the term electrophore from the Greek ήλεκτρον ('élektron'), and φέρω ('phero'), that is, carrier of electricity.

Other static electricity generators that use the same principle of electrostatic induction are the Wimshurst machine and the Van de Graaff generator.

Electrophore diagram

Electrophorus scheme

The purpose of the electrophore is to positively charge a disc of conductive charged material.

It is made up of:

• a disc of generally metallic conductor material, with an insulating handle to hold it (in the top drawing you see the handle, in the diagrams below is not represented);
• a sheet of insulating material. In the original version it was a resin cake;
• cat skin (in the original version), rabbit skin, or a simple well dry wool fabric, easier to get.

Operation

The electrophore works as follows:

1. First, the upper surface of the resin cake or the insulating foil is rubbed with the skin of cat or rabbit (on the side of the hairs), or with a wool fabric, so that the surface is charged negatively by friction. Once the insulation is loaded, the metal disc is approached holding it by the insulating handle (Figure 1), so both the drive disc, as the resin cake or the insulating sheet, are polarized, placing the negative loads of the conductor on the upper surface as a result of the repulsion carried by the negative loads that the insulating material has on its surface.
2. The drive is supported on top of the insulation, in contact. Since the insulator has excess negative load its potential is negative; as they are close, the potential of the neutral metal disc is also negative.
3. It connects the drive to the ground (if you don't have something that serves as a land grab, just touch it with a finger); as the land is at potential of 0 V and the metal disc has a negative potential, the disk tends to lose negative load. A negative load current (electronic control) originates from disk to earth, which ceases when the disk's potential is 0 V. This happens when the disk is loaded positively so that its positive potential is nullified with the negative generated by the insulating, so the total potential is 0 V. In other words, the potential of the superior face of the insulating material refers to soil.
4. The metal disc is disconnected from the socket, the electric potential remains 0 V.
5. The metal disc is separated from the insulator, holding it through the handle, since if the disc is touched with the hand, at the time the disc is removed from the insulating sheet, the potential of the disk would pass from 0 V to a positive potential, with what electrons of the body would pass to the disk, downloading it. This action to remove the disk from the loaded insulating material is the one that will induce a load of a few thousand volts on the metal disc.

Now the metal disk is positively charged; if it has a sufficient charge and a finger is brought near the disk without touching it, a spark will be seen to jump between the finger and the disk, which will thus be discharged.

The charge of the metal disk can be used for different experiments. For example, if the disc is placed in contact with an insulated conductor, it will be verified that the charge can be transported over a distance. Since the static charge that the dielectric base acquired by rubbing is not spent during the process of charging the conductive disk, it can be recharged many times without having to rub it again. This is so because the energy used to charge the disk is not supplied by charging the dielectric base, but by the mechanical work of separating the disk from the base. It is for this reason that Volta called it a perpetual electrophore. In reality, whether it is used or not, the charge on the dielectric base is slowly lost over hours or days, especially if there is high relative humidity, due to recombination with charged particles in the atmosphere. opposite sign.

The German scientist Georg Christoph Lichtenberg built in 1777 one of the largest electrophores ever made. It was just under two meters in diameter, and the metal disk had to be raised and lowered with a system of pulleys. Sparks up to 15 inches (nearly 400,000 volts) have been reported. Lichtenberg used these discharges to create strange, branching, tree-like drawings known as Lichtenberg Figures.

A few other insulating materials, such as certain special glass, can produce reverse polarity voltages.

Construction of an electrophore

An electrophore can be made from readily available materials. For the metal disc, a snap-on lid from a metal powdered milk container or similar (flat, without trim) can be used, with its rim facing up. As an insulating handle, a rod or tube of plastic material, for example the outer tube of a ballpoint pen, can be used, which can be adhered to the center of the disk with supercement (cyanoacrylate), with epoxy resin, or with another strong adhesive, such as a polymerizable putty of two components.

The original insulating material was a resin cake (material sold in hardware and chemical stores), but it can also be made with a large piece of sulfur (available in straw broom and chemical factories), chosen with a surface flat and larger than the metal disk. The voltages generated in this way will be around eight or ten thousand volts, which are not dangerous due to the low current that the electrophore is capable of generating. The length of the spark makes it possible to calculate approximately the electrical voltage reached at a rate of one thousand volts for each millimeter of spark.

Instead of the resin or sulfur cake, a sheet of plastic material can be used, for example acrylic, mylar, polystyrene, etc., but the stresses generated in this way can be lower. Instead of hard-to-find cat or rabbit fur, a well-dried woolen cloth can be used with the same result.

It is convenient to do the experiment only on days when the ambient relative humidity is less than 50 or 60%. To carry out the experiment, it is important to carefully follow the five steps indicated in the previous section; otherwise the electrophore will not work.