(E14) Early History of Electricity and Magnetism
(Much of this section is optional. Skim the descriptive parts if you are in a hurry, savor them to understand "where all this came from.")
So far this study has examined the flow of the "electric fluid," stressing aspects relevant to everyday life. But long before people knew of electrons and protons, they wondered--what did such a fluid consist of? A flow through a pipe usually consists of water, compressed air, hydraulic fluid, blood or some other fluid. What kind of material flows in an electric current?
They named it electric charge. When it is not flowing, we call it "static electricity" (static = not flowing).
Static electricity was already known to ancient Greek philosophers. Thales of Miletus, around 600 BC, probably knew that amber--fossilized pine-sap, a plastic-like non-conducting material--could attract light pieces of straw and feather when lightly rubbed with dry cloth or fur (most modern plastics also act that way--a plastic comb, for instance). Before discussing this further, however, it may help to examine permanent magnets, which share many of the features of static electricity.
The ancient Greeks also knew about magnets. They noted that on rare occasions "lodestones" were found in nature, chunks of iron-rich ore with the puzzling ability to attract iron. Some were discovered near the city of Magnesia (now in Turkey), and from there the words "magnetism" and "magnet" entered the language. The ancient Chinese discovered lodestones independently, and in addition found that after a piece of steel was "touched to a lodestone" it became a magnet itself.
Around the year 1000 (or earlier, according to some sources) the Chinese discovered another strange (but very useful!) property of magnets. Placed in a vessel floating in a bowl of water, a lodestone or a magnetized steel bar always lined up with its magnetic poles--the points where the magnetic pull was strongest--in the north-south direction. From that grew the magnetic compass, which helped sailors keep a straight course far from land and made possible voyages of discovery such as the ones by Columbus.
The study of nature by Greek philosophers (about 500 BC-200 AD) was followed by an era of little progress. Science again started advancing after the invention of the printing press by Johann Gutenberg (d. 1468), with the new astronomy of Nicolaus Copernicus (1473-1543), Galileo Galilei (1564-1642) and Johannes Kepler (1571-1630). The era of new ideas, art and instruments which followed became known as the renaissance, French for "rebirth."
One signpost of the new era was the book "De Magnete" (Latin for "On the Magnet") published in London in 1600 by William Gilbert, a prominent medical doctor and (after 1601) personal physician to Queen Elizabeth I. Gilbert's great interest was in magnets and the strange directional properties of the compass needle, always pointing close to north-south. He correctly traced the reason to the globe of the Earth being itself a giant magnet, and demonstrated his explanation by moving a small compass over the surface of a lodestone trimmed to a sphere (or supplemented to spherical shape by iron attachments?)--a scale model he named "terrella" or "little Earth," on which he was able to duplicate all the directional properties of the compass. (here and (here)
Even earlier it was shown that the attractive power of any magnet seemed concentrated at two "poles" at its end. Two types of poles existed, named for their role in the compass needle--the north-seeking pole ("N" or "north" for short) and the south-seeking pole ("S" or "south"). Two poles of different types were attracted, but poles of the same kind repelled each other.
If one cut a bar magnet in half, one might naively expect one half to contain the N pole and the other the S pole, but this never happens. Instead new poles appear at the break, leaving one with two shorter bar magnets, each with a matched pair of (N,S) poles. (Figure)
Also, the new poles are weaker. Gilbert correctly explained this observation by proposing that the sources of the magnetic force were distributed all over the substance of the magnet. The poles at the end were merely the points where that force emerged to the outside world. Each half of the broken magnet had such points, but since each pole channeled its force from only half as much magnetized material, that force was weaker.
How Iron is Attracted
Gilbert noted--as we all probably do, at one time or another--that while poles of two magnets may attract or repel, ordinary iron was always attracted. He was again right in guessing the reason: ordinary iron touching the pole of a magnet became itself a temporary magnet, with (N, S) polarities that favored attraction to the magnet (see below). As long as the temporarily magnetized iron touched the magnet, it could itself attract other objects, e.g. nails, and two nails hanging from neighboring points repelled each other. Soft iron lost its temporary properties as soon as it was removed, but steel could retain them, making it useful for compass needles.
Steel is an alloy of iron with 0.5-1% of dissolved carbon. Since iron is extracted from its ores by heating it with carbon (much of which is burned away while heating the mix), the molten iron first emerging solidifies to cast iron with about 4% of carbon. Cast iron is strong but too hard to work and somewhat brittle, as its large hard crystals may break apart. Blowing air or oxygen through the melt burns away carbon, leaving various grades of steel, or ending in non-elastic "soft iron" with little carbon, used in baling wire, fences and coat hangers.
If each magnet had two matched poles, why could one magnet attract another? Because the distances between the poles usually differ.
Suppose we place an iron bar (thin outline) next to a magnet with poles marked N and S (drawing). It now becomes a temporary magnet, with poles N' and S' of equal strength. Then the neighboring poles S and N' attract each other strongly, while the pairs (N, N') and (S, S'), being much further apart, repel each other much more weakly, not enough to overcome the strong attraction.
At great distances the forces are nearly equal, and the tendency is to rotate the magnet (as happens with a compass needle) rather than pull it closer.
One phenomenon which Gilbert found somewhat similar to magnetic attraction was the one found by Thales, that amber, glass, crystal and some other substances could attract light object when gently rubbed with cloth or fur, in a dry location. The Greek word for amber is elektron, so he named such materials "electricks" and their attraction the "electrick force." From that grew modern terms such as electricity, electric current, electrons and electronics.
Gilbert studied the strange force, finding (for instance) that while a thin layer of water disrupted electric phenomena, a thin layer of oil did not. He even constructed a "versorium," a lightweight nonmetallic pivoted needle resembling a magnetic compass, which obviously inspired it. Near an electrified object that needle would point in the direction of the "electrick force"--in what today would be called the direction of the electric field.
However, he did not realize that just as two varieties of magnetic poles existed, N and S, so electricity also came in two types, now denoted + and – (positive and negative electric charges). That was only found around 1733 by Charles Dufay in France, and like magnetic poles, charges of different types attracted each other and of the same type repelled.
One big difference existed, however: positive and negative charges could be separated. While every magnet carries N and S poles of equal strength, electrified objects could also carry one type of charge only. Rubbing glass created one type, rubbing amber the other. Still, the total charge was always conserved, so if glass rubbed with cloth became positively charged, the cloth itself always received an equally large negative charge.
The reason bits of straw are attracted by a charged object is very similar to the reason iron pins are attracted to the pole of a magnet. Just as a pin near a magnet develops temporary magnetic poles, so a piece of straw near a charge develops temporary electric ones.
Matter contains atomic-size electric charges, which we now know as electrons (negative) and atomic nuclei (positive, since they contain positive protons). In the straw, as in any other ordinary material, these are thoroughly mixed, so that any bit of matter contain equal amounts of positive and negative charge. As a result, it behaves as an uncharged object.
Near a + charge, however (drawing), the electrons are attracted and are slightly displaced in the direction of the attraction, while the nuclei are repelled. The result is that the end of the straw closest to the + charge acts like a negative pole and is attracted. Those extra electrons come from the other end, which thus is left with a net positive charge. That end is repelled, but being more distant, its repulsion is weaker and the attraction wins out.
In passing one may note that some materials--e.g. very good insulators like teflon, charged deep in their interiors--can keep + and – charges separated for a long time without external intervention. They are therefore the electrical equivalents of permanent magnets and are accordingly named "electrets." Electrets are nowadays used in high-performance microphones.
Conductors and Insulators
Gilbert and those who followed him throughout the next century knew of only a few substances which could be charged electrically--amber, glass, sulfur etc.--and even these, only when dry. The reason was found through the work of Stephen Gray (1666-1736) in England, and of others: in substances such as dry glass and ceramics, electric charges stayed in place. Other substances such as metals or water could also be charged, but their electrical charge tended to escape, unless they were separated from the rest of the world by materials of the first group.
The first group of substances, the ones who do not allow charge to move, are known as (electric) insulators. Today many more such materials are known, such as rubber, motor oil, glass and practically all plastics, e.g. polyethylene and nylon (polyester). Nylon shirts tumbled in the hot dry air stream of a clothes dryer tend to charge electrically by friction and to cling together, whereas cotton shirts do not.
On the other hand metals, carbon and watery solutions (especially the ones that contain dissolved electrolytes such as salt) freely allow electric charge to flow from point to point and distribute itself among all the locations it can reach. Such materials are called electric conductors. If we try to charge a metal rod held in one's fingers, the charge will spread to the fingers to the body, and from there, perhaps, through the leather soles of one's shoes to the ground and to the rest of the world. No observable charge remains on the rod, so any observation will conclude that it has "drained away." However, such a rod can be charged if we insulate it from our fingers with a piece of polyethylene sheet.