(E14) Early History of Electricity and Magnetism
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 flowed in an electric current?
Not sure wht it was, they named it electric charge. When electric charge is not flowing but stands still, 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).
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)
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.
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.
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 had 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.
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.
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 through 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.
Next Stop: E15. Static Electricity|
Author and Curator: Dr. David P. Stern