William Gilbert Compares Electric Bodies and Magnetic Bodies

William Gilbert's De Magnete, Book Two, part 1

This post is part of a series of posts about William Gilbert’s De Magnete (On the Magnet¹), which is composed of six books. This is the first post on Book Two. Here are the previous posts related to De Magnete.

• On Little Earths and Little Suns

• De Magnete, Nothing Less than the First Ever Work of Experimental Physics

• William Gilbert Writes about the Loadstone

• William Gilbert Examines Iron, Calls Aristotle's Earth Element Dead

Books Two through Six of De Magnete correspond to the five movements that Gilbert describes:

Now five movements or differences of movement are perceived by us: COITION [Book Two] (commonly called attraction), an impulsion to magnetic union; DIRECTION [Book Three] toward the earth's poles, and verticity of the earth toward determinate points in the universe, and the standstill there; VARIATION [Book Four], deflection from the meridian, — this we call a perverted motion; DECLINATION [Book Five] (inclination or dip), a descent of the magnetic pole beneath the horizon; and circular movement, or REVOLUTION [Book Six]. [p.73]

In the first half of Book Two, Gilbert goes to great lengths to distinguish between what he calls electric bodies and magnetic bodies, and the different kinds of attraction that they participate in. It had been known since antiquity that amber, when rubbed, attracts light bodies such as chaff, the seed coverings separated from the seed while threshing grain. Similarly, it had been known that loadstone attracts iron. Because there is an attraction in both cases, many philosophers over the centuries had written that these were the same phenomenon. It turns out, as we shall see in this post, that this is not the case.

The term electric was introduced by Gilbert as a derivation from the word electron, one of the Greek words for amber:

The Greeks call this substance ἤλεκτρον [electron], because when heating by rubbing, it attracts to itself chaff; whence it is also called ἄρπαξ [harpax], and from its golden color, χρυσοφόρον [chrysoforon]. [p.75]

Amber Room, Catherine Palace, 1917

As for the term coition, Gilbert also coined that term to describe the attraction between magnetic bodies. In Gilbert’s eyes, since magnetic bodies only directly attract each other if their opposite poles are facing each other, it is more appropriate to consider the attraction of two magnetic bodies as a form of coupling. If, for example, two loadstones are each placed in a boat, and the two boats in a body of water, the boats will spin around to correctly align opposite poles of the magnetic bodies therein before the two boats can move towards each other.

Right from the beginning, Gilbert writes that there is tremendous confusion as to the nature of these diverse attractions:

Great has ever been the fame of the loadstone and of amber in the writings of the learned: many philosophers cite the loadstone and also amber whenever, in explaining mysteries, their minds become obfuscated and reason can no farther go. [p.74]

Thus in very many affairs persons who plead for a cause the merits of which they cannot set forth, bring in as masked advocates the loadstone and amber. But all these, besides sharing the general misapprehension, are ignorant that the causes of the loadstone's movements are very different from those which give to amber its properties; hence they easily fall into errors, and by their own imaginings are led farther and farther astray.... for men still continue in ignorance, and deem that inclination of bodies to amber to be an attraction, and comparable to the magnetic coition. [p.75]

Gilbert begins his discourse on electric bodies by explaining that they go well beyond the amber and jet [lignite] known to the ancients:

For not only do amber and (gagates or) jet, as they suppose, attract light corpuscles (substances): the same is done by diamond, sapphire, carbuncle, iris stone, opal, amethyst, vincentina, English gem (Bristol stone, bristola), beryl, rock crystal. Like powers of attracting are possessed by glass, especially clear, brilliant glass; by artificial gems made of (paste) glass or rock crystal, antimony glass, may fluor-spars, and belemnites. Sulphur also attracts, and likewise mastich, and sealing-wax [of lac], hard resin, orpiment (weakly). Feeble power of attraction is also possessed in favoring dry atmosphere by sal gemma [native chloride of sodium], mica, rock alum. This we may observe when in mid-winter the atmosphere is very cold, clear, and thin; when the electrical effluvia of the earth offer less impediment, and electric bodies are harder: of all this later.

Furthermore, these bodies attract far more than just chaff:

These several bodies (electrics) not only draw to themselves straws and chaff, but all metals, wood, leaves, stones, earths, even water and oil; in short, whatever things appeal to our senses or are solid: yet we are told that it attracts nothing but chaff and twigs. [pp.77-78]

When Gilbert is writing about “electrical effluvia,” he is presupposing that the electrical attraction, which corresponds to what we now call “static electricity,” is the result of the flow of some minute particles. Today, we say that the electric bodies are charged negatively, i.e., electrons flow towards these bodies, as a result of their being rubbed. But Gilbert was writing in 1600, at the very beginning of the study of electricity.

Gilbert informs that readers that anyone can do some of the experiments that he describes, encouraging them to create their own versorium:

Now in order clearly to understand by experience how such attraction takes place, and what those substances may be that so attract other bodies (and in the case of many of these electrical substances, though the bodies influenced by them lean toward them, yet because of the feebleness of the attraction they are not drawn clean up to them, but are easily made to rise), make yourself a rotating-needle (electroscope — versorium) of any sort of metal, three or four fingers long, pretty light, and poised on a sharp point after the manner of a magnetic pointer. Bring near to one end of it a piece of amber or a gem, lightly rubbed, polished and shining: at once the instrument revolves. [p.79]

Here, Gilbert is proposing that science need not be in the hands of experts, and that ordinary self-educated people can participate in the scientific process. In keeping with this principle of the democratization of science, André Koch Torres Assis wrote a little book about experiments on electricity that can be done with readily available materials, and explains in detail how to create three different kinds of versoria, along with simple experiments that can be conducted therewith.²

Once one has a versorium, one can start experimenting with various materials to determine which ones are electrical bodies. Gilbert found many, but also found many that are not:

And thus very many bodies are said to attract, whereas the ground of their action is to be sought elsewhere. A large polished lump of amber attracts; a smaller piece, or a piece of impure amber, seems not to attract without friction. But very many electric bodies (as previous stones, etc.) do not attract at all unless they are first rubbed; while sundry other bodies, and among them some gems, have no power of attraction, and cannot be made to attract, even by friction; such bodies [anelectrics — non-electrics] are emerald, agate, carnelian, pearls, jasper, chalcedony, alabaster, porphyry, coral, the marbles, lapis lydius (touchstone, basanite), flint, bloodstone, emery or corundrum (mugris), bone, ivory; the hardest woods, as ebony; some other woods, as cedar, juniper, cypress; metals, as silver, gold, copper, iron. The loadstone, though it is susceptible of a very high polish, has not the electric attraction. On the other hand, many bodies (already mentioned) that can be polished attract when rubbed. [p.83]

Gilbert continues with a discussion of what exactly is the difference between magnetic bodies, electric bodies, and bodies that are neither. Gilbert, working with the principle that electric bodies attract as the result of the expulsion of effluvia when they are rubbed, distinguishes between materials that are more terrene or more watery:

As is plain to all, the earth's mass or rather the earth's framework and its crust consist of a twofold matter, a matter, to wit, that is fluid and humid, and a matter that is firm and dry. From this two-fold matter, or from the simple concretion of one of these matters, come all the bodies around us, which consist in major proportion now of terrene matter, anon of watery. [p.83]

Those bodies that are more watery are electric:

Hence all bodies that derive their origin principally from humors, and that are firmly concreted, and that retain the appearance and property of fluid in a firm, solid mass, attract all substances, whether humid or dry. [p.84]

Those bodies that are more terrene are magnetic:

Such as are parts of the true substance of the earth or differ but little from that, appear to attract also, but in a very different way, and so to speak, magnetically: of them we are to treat later. [p.84]

Finally, those that are a mix of terrene and watery are neither magnetic nor electric:

But those that consist of mixed water and earth, and that result from equal degradation of both elements — in which the magnetic force of the earth is degraded and lies in abeyance, while the aqueous humor, spoilt by combination with a quantity of earth, does not form a concretion by itself, but mingles with the earthy matter — such bodies are powerless to attract to themselves aught that they are not in actual contact with, or to repel the same. [pp.84-85]

For this reason it is that neither metals, marbles, flints, woods, grasses, flesh, nor various other substances can attract or solicit a body, whether magnetically or electrically (for it pleases us to call electric force that force which has its origin in humors). [p.85]

Gilbert also explains that some materials are not electric if too soft, but become electric should they harden:

But bodies consisting mostly of humor and not firmly compacted by nature wherefore they do not stand friction, but either fall to pieces or grow soft, or are sticky, as pitch, soft rosin, camphor, galbanum, ammoniacum, storax, asa, gum benjamin, asphaltum (especially in a warm atmosphere), do not attract corpuscles. For without friction few bodies give out their true natural electric emanation and effluvium. Turpentine resin in the liquid state does not attract, because it cannot be rubbed; but when it hardens to a mastic it does attract. [p.85]

But most importantly, Gilbert insists that there is something fundamentally different between the electrical and magnetic movements:

In all bodies everywhere are presented two causes or principles whereby the bodies are produced, to wit, matter (materia) and form (forma). Electrical movement come from the materia, but magnetic from the prime forma; and these two differ widely from each other and become unlike, — the one ennobled by many virtues, and prepotent; the other lowly, of less potency, and confined in certain prisons, as it were; wherefore its force has to be awakened by friction till the substance attains a moderate heat, and gives out an effluvium, and its surface is made to shine. [p.85]

So, with respect to magnetic movement, Gilbert assumes that rubbing an electric body creates some kind of effluvium, i.e., some kind of liquid discharge, of some unknown form, that is neither water nor air. In fact, the presence of humidity in the air prevents the flow of this effluvium. On the other hand, when there is attraction of a body by an electric body, no air moves, and this can be demonstrated by putting a candle near the two. The candle in no way moves. Furthermore, the closer a versorium is placed to an electric body, the faster it turns to point to the body:

Therefore the effluvium called forth by a friction that does not clog the surface — an effluvium not altered by heat, but which is the natural product of the electric body — causes unition and cohesion, seizure of the other body, and its confluence to the electrical source, provided the body to be drawn is not unsuitable by reason either of the circumstances of the bodies of of its own weight. The effluvia spread in all directions: they are specific and peculiar, and sui generis [of their own kind], different from the common air; generated from humor; called forth by calorific motion and rubbing, and attenuation; they are as it were material rods — hold and take up straws, chaff, twigs, till their force is spent or vanishes; and then these small bodies, being set free again, are attracted by the earth itself and fall to the ground. [pp.96-97]

Electric movement also differs from magnetic movement because in the latter case, both bodies participate in the attraction:

The difference (distinction) between electric and magnetic bodies is this: all magnetic bodies come together by their joint forces (mutual strength); electric bodies attract the electric only, and the body attracted undergoes no modification through its own native force, but is drawn freely under impulsion in the ratio of its matter (composition). [p.97]

Clearly, much of the above discussion on Gilbert’s part was speculative, and only with time and further research would some of these questions be further developed, if not necessarily solved. Nevertheless, the following difference is indisputable. Electric bodies attract other bodies in a straight line, while magnetic bodies only do the same when opposite poles of the bodies are directly aligned.

Bodies are attracted to electrics in a right line toward the centre of electricity: a loadstone approaches another loadstone a loadstone on a line perpendicular to the circumference only at the poles, elsewhere obliquely and transversely, and adheres at the same angles. The electric motion is the motion of coacervation [amassing, gathering] of matter; the magnetic is that of arrangement and order. The matter of the earth's globe is brought together and held together by itself electrically. The earth's globe is directed and revolves magnetically; it both coheres and, to the end it may be solid, is in its interior fast joined. [p.97]

After this brief discussion of electric bodies, Gilbert continues with the study of magnetism, as we will see in the upcoming posts.

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1. William Gilbert. De Magnete. Dover, New York, 1958. Translation by P. Fleury Mottelay of De Magnete, first published in 1600.

2. André Koch Torres Assis. The Experimental and Historical Foundations of Electricity. Montreal: Apeiron, 2010. Print-on-demand: http://www.amazon.com/dp/0986492639. PDF: http://www.ifi.unicamp.br/~assis/Electricity.pdf. Chapter 3: The Versorium.