<P> In systems of units other than SI such as cgs, electric charge is expressed as combination of only three fundamental quantities (length, mass, and time), and not four, as in SI, where electric charge is a combination of length, mass, time, and electric current . </P> <P> From ancient times, persons were familiar with four types of phenomena that today would all be explained using the concept of electric charge: (a) lightning, (b) the torpedo fish (or electric ray), (c) St Elmo's Fire, and (d) that amber rubbed with fur would attract small, light objects . The first account of the amber effect is often attributed to the ancient Greek mathematician Thales of Miletus, who lived from c. 624--c. 546 BC, but there are doubts about whether Thales left any writings; his account about amber is known from an account from early 200s . This account can be taken as evidence that knowledge the phenomenon was known since at least c. 600 BC, but Thales explained this phenomenon as evidence for inanimate objects having a soul . In other words, there was no indication of any conception of electric charge . More generally, the ancient Greeks did not understand the connections among these four kinds of phenomena . The Greeks observed that the charged amber buttons could attract light objects such as hair . They also found that if they rubbed the amber for long enough, they could even get an electric spark to jump, but there is also a claim that no mention of electric sparks appeared until late 17th century . This property derives from the triboelectric effect . In late 1100s, the substance jet, a compacted form of coal, was noted to have an amber effect, and in the middle of the 1500s, Girolamo Fracastoro, discovered that diamond also showed this effect . Some efforts were made by Fracastoro and others, especially Gerolamo Cardano to develop explanations for this phenomenon . </P> <P> In contrast to astronomy, mechanics, and optics, which had been studied quantitatively since antiquity, the start of ongoing qualitative and quantitative research into electrical phenomena can marked with the publication of De Magnete by the English scientist William Gilbert in 1600 . In this book, there was a small section where Gilbert returned to the amber effect (as he called it) in addressing many of the earlier theories, and coined the New Latin word electrica (from ἤλεκτρον (ēlektron), the Greek word for amber). The Latin word was translated into English as electrics . Gilbert is also credited with the term electrical, while the term electricity came later, first attributed to Sir Thomas Browne in his Pseudodoxia Epidemica from 1646 . (For more linguistic details see Etymology of electricity .) Gilbert was followed in 1660 by Otto von Guericke, who invented what was probably the first electrostatic generator . Other European pioneers were Robert Boyle, who in 1675 stated that electric attraction and repulsion can act across a vacuum; Stephen Gray, who in 1729 classified materials as conductors and insulators . In 1733 Charles François de Cisternay du Fay, inspired by Gray's work, made a series of experiments (reported in Mémoires de l'Académie Royale des Sciences), showing that more or less all substances could be' electrified' by rubbing, except for metals and fluids and proposed that electricity comes in two varieties that cancel each other, which he expressed in terms of a two - fluid theory . When glass was rubbed with silk, du Fay said that the glass was charged with vitreous electricity, and, when amber was rubbed with fur, the amber was charged with resinous electricity . Another important two - fluid theory from this time was proposed by Jean - Antoine Nollet (1745). In 1839, Michael Faraday showed that the apparent division between static electricity, current electricity, and bioelectricity was incorrect, and all were a consequence of the behavior of a single kind of electricity appearing in opposite polarities . It is arbitrary which polarity is called positive and which is called negative . Positive charge can be defined as the charge left on a glass rod after being rubbed with silk . </P> <P> One of the foremost experts on electricity in the 18th century was Benjamin Franklin, who argued in favour of a one - fluid theory of electricity . Franklin imagined electricity as being a type of invisible fluid present in all matter; for example, he believed that it was the glass in a Leyden jar that held the accumulated charge . He posited that rubbing insulating surfaces together caused this fluid to change location, and that a flow of this fluid constitutes an electric current . He also posited that when matter contained too little of the fluid it was negatively charged, and when it had an excess it was positively charged . For a reason that was not recorded, he identified the term positive with vitreous electricity and negative with resinous electricity . William Watson independently arrived at the same explanation at about the same time (1746). </P>

Who discovered positive and negative charges in atoms