Electrostatics

Great ancient civilizations of the Mediterranean were interested in the phenomena related to electricity. Legend has it that around 900 BCE, Magnus, a Greek shepherd, walked across a field strewn with black stones that “magically” extracted iron nails from his sandals. It happened in a region nowadays called Magnesia, although according to other mythological stories, the name of the land derives from the name of its first king, Magnes – the son of Zeus and Thyia.

 

When mentioning Greek mythology, one cannot overlook Prometheus who first created man from clay mixed with tears and then equipped him with fire and taught him how to use it. He also bequeathed to man the rest of the knowledge on which the progress of civilization was based. Thanks to him people learned how to build shelters, write, process metals and harness and use the forces of nature.

 

 

Finally – around 600 BCE – the first documented experiments were carried out in the field of electrostatics. They were performed by Thales of Miletus, who made some observations about amber (electron in Greek – which gave rise to the now common term “electricity”) that, when rubbed, starts pulling on other objects. Although Thales was not able to explain the phenomenon, or there was no chance to use it in practice, it drew the attention of contemporary people to natural phenomena, arousing increased interest in them.

 

However, over the following twenty four centuries knowledge concerning electricity did not progress too much. The situation changed only in the period of Enlightenment when natural sciences were liberated from the paralysing influence of church dogma and, as a result, began to develop rapidly. Already in the final years of the Renaissance, in 1600, an English physicist, William Gilbert, published his treatise “On the Magnet and Magnetic Bodies, and on the Great Magnet the Earth” in which the term “electricity” was used for the first time. He used innovative terms such as: magnetic field, forces and electrical attraction to describe magnetic and electrostatic properties of various bodies. Earlier, in 1551, Girolamo Cordano – an Italian mathematician – noticed that the properties of amber and magnetite were actually two different things. Scientist began to distinguish between magnetism and electricity although there still seemed to be an obvious relation between them.

 

Scientific progress in the following years of the seventeenth century and throughout the eighteenth century was surprising. In 1660 the mayor of Magdeburg, Otto von Guericke, invented the first electrostatic machine. It was a rotating glass globe coated with sulphur, which was charged when rubbed by hand. In 1705, Francis Hauksbee conducted a series of experiments where he tried to establish reasons why light appeared on the glass tubes in Torricelli’s barometers. First, this phenomenon was associated with the mercury the device contained, but soon it turned out that light appeared on glass surfaces regardless of whether the vessels were empty or not. Hauksbee observed that sparks appeared on objects made of glass when they were placed near other objects that had been electrically charged. He organised breathtaking demonstrations with the aid of numerous artefacts of various shapes emitting light in a full range of colours.

 

In 1729, Stephen Gray discovered that electric charges “travelled” through some materials while in others they did not. He used wet packthread to transmit electric current over a distance of 765 metres, and established that silk thread did not have such properties. Eventually, at the final stage of his experiments he transmitted a charge between an electrostatic machine and an electroscope over a distance of 88 metres through a wire suspended on silk threads. Thus, electrical conduction was discovered and the basic design principles (placing a conductor on insulators) were determined for all electricity transmission lines, both these used for power supply and telecommunications.

 

Five years later, a retired officer, Charles du Fay announced the findings of his experiments to the French Academy of Sciences. First, he refuted the previous classification of “electric” and “non-electric” bodies put forward by Jean Desaguliers. Du Fay proved that all bodies exposed to electricity can be electrically charged! His other discoveries were related to conduction, the electric properties of a human body insulated from the ground. However, his most significant achievement was the introduction of the division into two types of electricity: vitreous (generated on glass, stones, noble metals and animal fur) and resinous (generated on amber, resin, sealing wax and paper). He also observed that charges of the same kind repelled each other whereas different charges attracted each other. This differentiation still holds true today except that we use the terminology introduced in 1752 by Benjamin Franklin who defined charges as “positive” and “negative”.

 

 

A breakthrough came with the Leiden jar – the first capacitor – designed in 1745, independently by two people: Pieter van Musschenbroek at the University of Leiden (Leyden) and Ewald Jürgen Georg von Kleist in Kamień Pomorski. The glass was an insulator between two layers of metal foil covering the inner and outer surfaces of the jar. The jar was filled with water and closed with a plug through which a copper wire passed.  However, the most important experiment with that device was conducted by Benjamin Franklin who, in June 1752, used a kite to charge the Leiden jar with electricity from lightning. The American scientist proved that lightning is an electrical discharge, which was very important in practice since it enabled finding scientific means of protection against the effects of lightning strikes. Franklin's other experiments and discoveries, even if they were less spectacular, were also important for progress in the utilization of electricity. He observed that glass rubbed with a cloth was electrified with a charge having the same force but opposite potential as the cloth. Thus, he further concluded that there must be rather one type of electricity instead of two (as du Fay, Watson and others had claimed before), and where when two objects are rubbed against each other one of them receives the surplus while the other is deficient in electricity, so afterwards they tend to offset the difference. To demonstrate his findings, Franklin had two people stand on isolated platforms and rub glass with a cloth. Then, one of them touched the glass while the other touched the cloth. When they drew the palms of their hands close to each other, a strong spark jumped between their fingers. The charges with which their bodies had been charged were completely neutralised. 

 

As early as 1748 Benjamin Franklin demonstrated a pre-prototype of an electric motor by fastening two metal knobs diagonally to a round disc made from an insulator. Afterwards, he installed it between similar balls facing one another so that the disc could turn and the distance between the knobs at their closest position was very small. When the balls on the stand were electrically charged – one with a positive and the other with negative charge – the wheel was set in motion. The balls on the disc were attracted by those on the stand and when their spacing reached the minimum, a spark-over occurred between them, thus charging them identically. At that moment the balls started pushing off one another, thus setting the disc into motion again. In 1753, the Royal Society awarded Benjamin Franklin the Copley Medal – the highest scientific distinction in England at that time.

 

By far the most important event of that period was Charles Augustin de Coulomb’s publishing of the fundamental law of electrostatics in 1784. Similar conclusions had previously been reached by other scientists (John Michell, Johann Tobias Mayer, and Henry Cavendish), but they had not fully proved their theorems. Coulomb used a self-designed torsion balance and stated that the force of interaction between two charged particles was directly proportional to the product of their charges and inversely proportional to the square of the distance between them. From that time Coulomb's law had made it possible to determine the forces of interaction in any arrangement of electric charges and enabled further research and development in the area of electrical science.

 

The close of the eighteenth century saw experiments carried out by an Italian anatomist, Luigi Galvani. When studying frogs he noticed that the muscles of a dead animal twitched when touched by metal. “Animal electricity” had long been a subject of experiments. In the 17th century similar nerve and muscle responses were described by a Ducthman, Jan Swammerdam, and in 1700 they were observed by Leopold Caldani in Italy. In Berlin in 1762, Johann Georg Shulzer published his discovery that two pieces of different metals – silver and lead – placed on a tongue combined to produce a taste similar to that of ferrous sulphate. In addition, the effect could not be felt when one piece was on the top of the tongue and the other piece under the tongue. It could be felt only when their outer edges were in contact. However, no one had yet managed to formulate a satisfactory explanation of those phenomena and connect all those discoveries to form a coherent body of knowledge. It was only Galvani, through his experiments focusing on electrical properties, who proved that electricity was the “force” causing the muscles of dead animals to twitch. Galvani published his findings in 1791, after eleven years of experiments, in a work entitled De viribus elictricitatis in motu musculari commentarius (Commentary on the Effect of Electricity on Muscular Motion).

 

 

One of those who became interested in Galvani’s work was Alessandro Volta – a natural history professor at the University of Pavia, who was convinced that the metals used in the experiments were the actual source of electricity. The resultant intense disagreement between the scientists was brought to an end by Galvani’s death in 1798. Two years later, in his letter to the Royal Society in London, Volta reported that he had designed a “pile” that behaved similar to a Leiden jar because it made it possible to store energy.

 

However, the general difference was that the Leiden jar after discharge had no electric charge at all while the voltaic pile continuously renewed the charge. Volta’s device was composed of eighty alternating zinc and copper discs separated by cardboard, felt or leather spacers soaked in salt water or acid solution. It was placed in a vertical stand with wires at the bottom and top. Volta considered his invention to be an experimental proof that electricity was generated as a result of contact between two different metals – he believed it would be his final victory in his dispute with Galvani.

 

Experiments conducted by other scientists based on Volta’s description proved that the source of electricity was chemical reactions and the theory of metallic contact was erroneous. Taking Volta’s research further, in1801 Johann Ritter proposed a classification of metals with regard to the force of electric current they generated when submerged in salt or acid solutions. Volta developed the same concept slightly later and made a table, known as “Volta’s electrochemical series”, in which every metal has positive potential compared to that in the following line. Thus, the concept of electromotive force was formulated although still a quarter of a century had to pass before Ohm published his law. 

 

New cells and their assemblies were built rapidly and were referred to as batteries. Not aware of the full nature of electricity, researchers obviously focused on the side, but at the same time more spectacular, effects of electric discharge such as noise and sparks. The analogy to military terminology was almost natural and nowadays terms such as (electric) “charge” or “battery" – a word borrowed from artillery – are still in use.

 

At the beginning of the 18th century electricity was known to the elite and scientists only. The application of the new inventions was very limited. Firstly, batteries provided small amounts of energy and their life was short. Secondly, besides the obvious and perceptible effects of electricity such as the heating or even burning of conductors, there was no practical use for them. 

 

Electrical generators were yet to be invented and a few steps had to be made to achieve that. One of these steps was the research by Hans Christian Ørsted. During a lecture on the thermal effects of the flow of electricity he noticed that the needle of a compass deflected towards the conductor when the latter carried electricity. Over the following months he studied the phenomenon and discovered that not only did the electricity influence the magnetised needle but also the magnet could affect a moveable wire carrying electric current. Ørsted published his findings on 21 June 1820 in Italian newspapers thanks to which his research became widely spread and popular. Electromagnetism described by the Danish scientist became the key to subsequent discoveries and inventions.

 

 

In the same year, André Marie Ampère delivered a series of lectures at the French Academy presenting new discoveries in electromagnetism. He noticed that electromagnetic forces occurred not only between the conductor and the magnet but also between two conductors carrying electric current. Next, he defined the relationship between the direction of the current and the deflection of the compass needle and demonstrated that parallel wires attract one another when carrying electricity in the same direction and repel one another if the current flows in opposite directions. Based on the above concepts, he concluded it was possible to construct an “electric magnet”, and then designed a prototype: a copper wire coil that upon connecting it to electricity exhibited identical properties to those of a permanent magnet. Ampère also suggested that such instruments should be used to transmit information over distance – a concept of a telegraph emerged. We owe to him the division of electrical sciences into electrostatics and electrodynamics. Apart from Coulomb’s torsion balance, scientists of the time did not have any instruments to carry out electrical measurements except for those that can only be called indicators today. The publication of Ampère’s basic law made it possible to use galvanometers for electrical measurements. Ampère’s law published in 1822 described the relationship between magnetic induction around a conductor and the strength of the current carried by the conductor.

 

Georg Simon Ohm began his studies on electricity in conductors in 1825 and two years later he published his findings including a law named as Ohm’s law. It describes the proportionality of voltage to amperage of current carried by a conductor with specified resistance.On 3 September 1821 Michael Faraday proved that a conductor connected to an electric current rotates in a magnetic field, similar to the needle of a compass changing its position in an electric field. He conducted his first experiments using a metal vessel filled with mercury. In the centre of the vessel was a vertical magnet stick with the tip protruding outside the metal. Above the vessel he suspended a conductor, one tip of which was immersed in mercury. After the current was switched on, it started rotating around the magnet. Faraday used the same set in his second experiment, only the conductor was immobilized and it contacted the central point of the mercury pool, and the magnet was replaced by a magnetic needle having one tip coated with platinum to increase its weight. The needle moved freely inside the vessel touching the bottom with one tip, while the other tip protruded above the surface. And again – when the wire was connected with the cell, with the flow of current – the needle started rotating around the conductor. Soon Faraday also noticed that a metal disc on an axis mounted between the poles of a U-shaped magnet starts rotating when electric current flows through the axis. A similar experiment was successfully conducted some time earlier by Peter Barlow, and this motor prototype was referred to as "Barlow’s wheel”.

 

However, it was Michael Faraday who completely devoted his attention and time to investigating issues related to electromagnetism. In 1822 he wrote down in his notebook: “convert magnetism to electricity” and aimed for this goal over the following years. He finally succeeded nearly ten years later, in 1831, when he carried out a series of experiments using a coil with one of its tips connected to a galvanometer. Faraday observed the readings of the instrument and noticed that when the magnet was moved in and out the needle of the meter deflected to opposite sides, and when the magnet was held still, the needle returned to its original position. Michael Faraday submitted the results of his experiments to the Royal Society on 24 November 1831, and the news on a new, sensational discovery created a stir among all scientists. The way to convert mechanical energy into electrical energy was already determined, although some time had to pass before effective instruments making it possible to put Faraday’s discoveries into practice were invented.