The electric vocabulary

Since the 1600s, the developments in the understanding of electrical phenomena have frequently altered the models and metaphors used by physicists to describe and explain their experiments. However, to this day, certain relics of past theories still drench the vocabulary of the subject, serving as distracting fog for future students. This article attempts, through historical illumination, to shine through the mist of electrostatic terminology and offer a clearer view of the classical model of electricity. Thales (624BC–546BC) The Greek philosopher Thales is documented as being one of the first to observe that a piece of amber, when rubbed with fur, would pick up small pieces of straw 1. This seems to have been amber’s defining characteristic for many centuries. For example, the Arabic word for amber, ‘ ’, (pronounced kah-roba) means ‘straw robber’. One Ancient Greek word for amber was ‘harpax’ meaning ‘robber’ [2]. However, from Homer’s ‘Odyssey’, and some other sources, we know that the Greeks also called amber, ‘ ’ (elektron). This derives from the word ‘elektor’ meaning ‘sun’s glare’, in reference to amber’s shiny surface [3]. Obviously, it is this word for amber that has carried into our modern vocabulary as ‘electron’. As well as amber’s ability to attract straw, Thales also knew that ‘lodestone’, a type of rock, attracted iron nails. Thales was very interested in these two tricks of nature. He likened the power that these objects had over other items (such as nails and straw) to the power humans and gods have to move and shape their world. This led him to a bold idea concerning lodestone and amber that he applied to the whole cosmos. Aristotle and Diogenes Laërtius write: 1 Very little is know of Thales. For a brief account, see [1]. ‘Thales, too, to judge from the anecdotes related of him, conceived soul as the cause of motion, if it be true that he affirmed the lodestone to possess soul, because it attracts iron’ [4]. ‘. . .Aristotle and Hippias say that he attributed souls also to lifeless things, forming his conjecture from the nature of the magnet, and of amber’ [5]. Thales’ ‘explanation’, by present standards, is underwhelming. The agency he proposes to explain the abilities of amber and lodestone (the property of having a soul) lacks any quantitative description, or prediction of additional phenomena. There is no experiment that could be set up to test whether Thales was incorrect, at least none we know of. Thus, his theory is not falsifiable and therefore is not one of modern science [6]. Nevertheless, this was the very first step of the human investigation into the behaviour of these two intriguing objects. William Gilbert (1544–1603) and Sir Thomas Browne (1605–82) Nature had to wait around 2200 years before any further investigation was made into amber or lodestone. In 1600, the English physicist 78 P H Y S I C S E D U C A T I O N 47 (1) 0031-9120/12/010078+09$33.00 © 2012 IOP Publishing Ltd The electric vocabulary William Gilbert, wrote a book called ‘De Magnete’ (‘On Lodestone’), considered to be one of the first modern scientific works. He was devoted to utilizing experiments as a way of finding out about the world, and this is why he was able to discover so many new things. The book, as the title suggests, is mostly concerned with lodestone. Gilbert observed that a piece of iron could behave like lodestone when the two were rubbed together. He found that there were two sides to a piece of lodestone which sometimes attracted other lodestones, and sometimes repelled, depending on which way around the two were held. Other investigations showed that a compass needle on a sphere of lodestone seemed to mimic where the needle points on the surface of our spherical Earth. Was the Earth a large ball of this special metal, Gilbert wondered 2. In addition to lodestone, Gilbert devotes a small section of the work to amber [8]. He lists a great many substances that are amberlike in their attractive properties and names them ‘Electricks, things which attract in the same manner as amber’ [9]. He also lists materials, other than straw, which are attracted towards electriks. In Gilbert’s original Latin text, he calls the electriks ‘electricita coporea’ (electric bodies) [10]. This was later anglicized to ‘electricity’ by Sir Thomas Browne in 1646. 3 Browne writes: ‘Again, The concretion of Ice will not endure a dry attrition without liquation; for if it be rubbed long with a cloth, it melteth. But Crystal will calefie unto 2 The original work ([7]) is a little verbose by modern standards. For a more condensed description of Gilbert’s findings read [12]. 3 Unfortunately, many textbooks, teachers and students use ‘electricity’ as a thick brushstroke term to be applied to all manner of electrical phenomena. Current, charge, potential difference, electrical power each all seem, to someone, to be ‘electricity’. However, Gilbert and Browne were thinking in more simple terms. Less was known about electrical phenomena in their time. ‘Electrics’ referred to amberlike objects and ‘electricity’ was the property they exhibited. Similarly, we have ‘elastic’ objects that have the property of ‘elasticity’. ‘Electricity’, in our time of greater understanding, is a word to describe a larger process. Just as ‘evolution’ as described by Darwin never names an item of the process as ‘evolution’, we do not call any aspect of electrical phenomena ‘electricity’. Electricity is nothing but the name of the textbook. That this word is misused by a great majority suggests that the electric vocabulary is widely misunderstood. electricity; that is, a power to attract strawes and light bodies, and convert the needle freely placed’ [11]. Gilbert was also the first person to draw a clear distinction between electrik (amber) and magnetik4 (lodestone) phenomena, which had been paired since Thales as similarly mystical objects. He does this in several ways. Firstly, he notes that amber requires rubbing by friction to reveal its hidden attractive abilities whereas lodestone does not. Also, the attraction of an electrik seems to depend upon the heat and moisture content of the surrounding air. This was not the case for magnetiks, which would still attract and repel under water [12]. Charles Du Fay (1698–1739) and Jean-Antoine Nollet (1700–70) Charles Du Fay was a French physicist who, in 1736, greatly developed the understanding of electriks. Inspired by Stephen Grey’s experiments at England’s Royal Society, which he had studied in detail, Du Fay carried out a large number of his own experiments with different substances [13]. In addition to the electrik substances named by Gilbert, Du Fay was able, with a combination of heating and rubbing, to turn almost all substances (except for metals and fluids) into electriks. Moreover, he noticed that these objects sometimes repelled as well as attracted and attempted to describe this previously unnoticed phenomena. ‘. . .if [a body] attracts [an electrified silk thread], ‘tis certainly of that kind of Electricity which I call vitreous; if on the contrary it repels, ‘tis of the same kind of Electricity as Silk, that is, of resinous’ [14]. So, Du Fay added a further distinction; not only could objects become ‘electrik’ (amber-like), but he found two categories. Objects that repel electrified glass he named ‘vitreous’ (from the 4 The word ‘magnetism’ comes from the mythical story of the discovery of lodestone. A man (some claim called ‘Magnes’) was sheparding in an area of Greece called Magnesia around 2000BC when he found that the nails in his sandals and the bottom of his staff stuck to the rock. This type of rock was later named lodestone. More generally, this type of thing, a ‘magnet’, is named from the Greek ‘magnı́tis lı́thos’ ( ), which means ‘Magnesian stone’. January 2012 P H Y S I C S E D U C A T I O N 79