Electromagnetic Theory and Light - Sci-Philately: A Selective History of Science on Stamps

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Electromagnetic Theory and Light

	Electric charge is a fundamental property of all matter, which consists of atoms, and it has existed since the beginning of the universe. One of its most spectacular visible manifestations is lightning, a discharge of electricity between clouds or between a rain cloud and the earth. The discharge heats the air molecules which are pushed apart with explosive force, creating a booming sound, or thunder. We see a lightning flash almost instantly, but the sound of thunder may trail the flash by seconds, since the speed of sound is much slower reaching us than the speed of light.
	Thales of Miletus (625?-546?BC) was a natural philosopher who investigated the nature of matter and the universe in all its states. His hypothesis was that all things came from water and ultimately returned to it. He was the founder of the Milesian school of philosophy and considered one of the Wise Men of Ancient Greece. While no writings by Thales exist, he is said to have predicted a solar eclipse in 585 BC, which so frightened the Medes and Lydian armies engaged in battle that the warring factions ceased fighting and made peace. He also recommended navigating by Ursa Minor, the constellation which contains Polaris. On the Greek stamp at left Thales is commemorated as the discoverer of static electricity generated when a piece of amber is rubbed with fur: it is then able to attract light objects such as straw or feathers. This effect was called "electricity" by William Gilbert in 1600, after elektron, the Greek word for amber.
	Abu Ali al'Hasan ibn al'Haitam is known in the West as Alhazen, born 965 in Persia and dying in 1039 in Egypt. He is called the Father of Optics for his writings on and experiments with lenses, mirrors, refraction and reflection. He correctly stated that vision results from light that is reflected into the eye by an object, not emitted by the eye itself and then reflected back, as Aristotle believed. He solved the problem of finding the locus of points on a spherical mirror from which light will be reflected to an observer. From his studies of refraction he determined that the atmosphere has a definite height, and that twilight is caused by refraction of solar radiation from beneath the horizon. The optical diagram on this Pakistani commemorative in blue, green and black is hard to decipher because of the lack of contrast.
	Benjamin Franklin (1706-1790), sometimes called the American Newton, was an inventor, publisher of pamphlets, maps, and broadsides, and experimenter besides being one of the country's founding fathers and first Postmaster General. We owe to Franklin the idea of positive and negative charge, and the law of conservation of charge. Famous for his daring kite experiment, in which he drew electricity from thunder clouds to a key suspended from the kite string, and thence to a Leyden jar, he also investigated the direction and velocity of the Gulf Stream.
	Prokop Divis (1698-1765) was a Czech scholar who studied and experimented with atmospheric electricity. He attempted to draw electricity from clouds and built a working lightning conductor, the first to offer actual protection, in 1753.
	Luigi Galvani (1737-1798) was an Italian anatomist and physician who, like many contemporary investigators, performed electrical experiments . He found that severed frogs' legs twitched when near a spark generator, and also when they came in contact with two different metals. Galvani concluded that the muscles were a heretofore unknown source of electricity, animal electricity. He was sadly disappointed when somewhat later Volta showed, in constructing his pile, that that the frog legs were not necessary to create a spark, rather, that two different metal s sufficed when separated by moist cardboard. Still, Galvani is credited with the original observation of this phenomenon.
	Alessandro Volta (1745-1827) was an Italian physicist whose important invention was the voltaic pile, an antecedent of the storage battery. He found that an electric current can be produced when chemicals come in contact with metals. This is the basis of electrolysis. Volta constructed his pile from alternating layers of copper and zinc discs in a salt water solution. Galvani had already discovered a current passing between two metals touching a muscle. Volta dispensed with the muscle and found that a current could still be produced. The electric unit volt is named in his honor. The stamp at left shows the voltaic pile on the right.
	The connection between electricity and magnetism was first demonstrated by Hans Christian Oersted (1777-1851). He brought a magnetic needle near a wire which was carrying an electric current. The needle was deflected at right angles to the wire, as is shown on this Danish stamp.
	The 1994 German stamp at left shows a resistor, a common element in electronic circuits, and commemorates the discovery of Ohm's law, named after the German mathematician Georg Simon Ohm (1787-1854) who formulated it. Ohm aspired to be a university professor, and after many teaching positions as a mathematician began to experiment in physics. His law describes the relationship between current, voltage, and resistance in a circuit, namely, that a voltage drop across a resistor is equal to the resistance times the current running through it, or V =R x I. The unit of resistance is also called an ohm; one ohm causes a one volt drop for a current of one ampere. Resistors are color-coded to show their value, or amount of resistance: the band at far right refers to the tolerance, or accuracy, and may be gold or silver, (5% or 10% accuracy). The two bands at the left indicate two numbers, which together form a value, such as 27 in this case; (red = 2, violet = 7). The third band is the code for a multiplier in powers of 10, in this case yellow, which stands for 10,000. The value for this resistor is therefore 270,000 ohms, but the tolerance value is somewhat in doubt, being neither distinctly gold, nor silver, though the yellow-greenish tinge makes gold more likely.
	Andre Marie Ampere (1775-1836) was inspired by the work of Oersted to study the interactions between current-carrying wires, and derived a mathematical formula describing this force. He devised the right-hand rule to describe current-flow. The unit of current, Ampere, is named after him.
	Dark spectral lines on the solar emission continuum are caused by absorption of radiation in the outer, cooler layers of the solar atmosphere. Although observed before, they were first mapped by Joseph von Fraunhofer (1787-1826), German physicist and inventor of the transmission grating. This illustrative stamp shows the Fraunhofer lines in the solar spectrum, which approximates that of a black body with 5800 degree K effective temperature peaking in the yellow, as we see each sunny day.
	Michael Faraday's (1791-1867) interests in electricity and magnetism were wide and varied. Observing the alignment of iron filings around a bar magnet, he decided that they followed lines of force of a magnetic field. He induced a current in one of two wire coils wrapped around a metal ring by passing a current through the other coil, creating a transformer. His ideas about force fields were later expressed mathematically by Maxwell as Maxwell's equations. As a chemist Faraday liquified carbon dioxide and chlorine and discovered benzene.
	Hermann von Helmholtz (1821-1894) was a protege of Humboldt's distinguished for his research in optics and the physiology of vision and acoustics, the law of conservation of energy, and electrodynamics. He made important discoveries about the structure and mechanism of the human eye and invented the opthalmoscope. He suggested the investigation of light beyond the visible range to his student Heinrich Hertz.
	Gustav Robert Kirchhoff (1824-1887) was a German physicist who worked with Bunsen on the development of the spectroscope and the measurement of spectra of the elements. He discovered that sodium emits the bright yellow lines appearing in absorption in the solar spectrum as observed by Fraunhofer; this suggested the possibility of investigating the composition of stars by spectrographic methods. His work in electricity is noted on this stamp: the sum of potential drops around a closed circuit loop is zero.
	James Clerk Maxwell (1831-79) derived mathematically the four equations that describe the interaction and relationship between electricity and magnetism as already observed by Faraday, and which are the foundation of electromagnetic theory. Maxwell's equations are the basis of all modern communication: they apply to all electromagnetic radiation, be they TV, radio, radar, light, or x-rays. The Nicaraguan stamp shows a transmission tower superimposed on a dish antenna and spherically emanating waves. Maxwell shares honors with Hertz on the Mexican stamp which commemorates global telecommunications.
	Heinrich Hertz (1857-1894) devised an oscillating electrical circuit ending in two metal spheres. The oscillations caused sparks across the air gap between the spheres, and Hertz was able to detect and map the electromagnetic fields generated, as predicted by Maxwell's equations. German and Czech stamps note the centenary of his research, showing the spark gap and the field lines.
	Albert Michelson (1852-1931) was the first American to win a Nobel prize (physics, 1907) but has not been recognized for this achievement by the U.S. Postal Service. A Swedish stamp gives us his small profile. His studies of the velocity of light with ever greater accuracy were based on a modification of Foucault's rotating mirrors. Light, as a wave motion, was then supposed to require the presence of a medium for its propagation, and this invisible substance was called the luminiferous ether, through which the earth also moved. It was supposed that light would have different velocities when moving perpendicular and parallel to the hypothetical ether, being "dragged" in the former case. By an ingenious experiment of splitting light beams and bringing them back together in his interferometer, Michelson and his associate Edward Morley, a chemist dedicated to producing high precision instruments, were unable to detect any difference in time traveled for the split beams, both parallel to and with the ether. This failed experiment obviated the need for the ether's existence, and proved that the velocity of light is constant, an assumption later made by Einstein in his general theory of relativity.
	Nikola Tesla (1856-1943) was an electrical engineer born in Yugoslavia. He developed the first alternating current induction motor after becoming a U.S. citizen. The Niagara Falls Power Company was the first to adopt his invention in the exploitation of hydro power, proving the universal adaptability of alternating current.
	Guglielmo Marconi (1874-1937) was an Italian engineer and physicist who recorded the first transatlantic transmission of radio signals. Hertz before him had already transmitted and received radio signals in the microwave range over short distances. Marconi received the 1909 Nobel prize in physics for his achievement.
	Chandrasekhara Venkata Raman (1888-1970) was an Indian physicist and Nobel Prize winner who discovered that light scattered by molecules in solution had some components changed in wavelength and that these changes were characteristic of the target molecules. These Raman spectra can be used to investigate and identify molecular structure. Such a spectrum appears next to Raman's portrait on this Indian stamp
	The Scottish physicist Robert Alexander Watson-Watt (1892-1973) is credited with the development of the first workable radar system in the 1930's, although there was concurrent research in other countries. Based on the reflection of shortwave radio waves, radar pinpoints a moving object's location by measuring the time elapsed between emitted and reflected pulses as well as their direction.
	German-born Canadian physicist and molecular spectroscopist Gerhard Herzberg (1904-1999) received the Nobel prize in Chemistry in 1971 for his contributions to molecular spectroscopy and investigation of free radicals, short-lived intermediate products in chemical reactions. Herzberg's techniques were fundamental in discovering the electronic structure and changing geometry of free radicals, which only exist for a fraction of a second while a reaction is taking place and offer insights in how it proceeds.
	Spectroscopy plays an integral part in modern forensic science in the identification of substances. Spectra are featured on this Canadian stamp commemorating the 100th anniversary of the Royal Canadian Mounted Police.

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