Metric Units and Postage Stamps Donald W. Hillger, Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO 80523-1375

he has evolved SI Unit Symbol Compound Quantity Person Units from the French (“”) T A André-Marie Ampère units of 1790 to be officially recog- (1775-1836) nized by the Eleventh General K base unit thermodynamic Lord Kelvin/William Conference on Weights and Thomson (1824-1907) Measures as the International °C K * System of Units (SI) in 1960. This Celsius temperature (1701-1744) note brings together brief biograph- ical notes of the scientists for whom Hz 1/s Heinrich Rudolf Hertz (1857-1894) many SI units are named, along 2 with reproductions of postage Nkg m/s stamps from around the world that (1642-1727) have been issued to honor these Pa N/m2 , stress men. This combination of science, (1623-1662) metrics, and history may also pro- JN menergy, , duce the bonus of introducing quantity of (1818-1889); no stamp someone to the hobby of topical W J/s , radiant stamp collecting. (1736-1819) Currently, 19 SI units are named CA s electric , Charles Coulomb after scientists (see Table I). Each SI quantity of (1736-1806) symbol is unique in its spelling VW/A (usually a capital first letter of the difference, (1745-1827) person’s name), and each unit is unique scientifically, describing a F C/V electric quantity to be measured using that (1791-1867) SI unit. (Celsius alone is ⍀ V/A electric resistance Georg Simon Ohm capitalized.) (1787-1854) By far the largest number of SI S A/V electric conductance postage stamps feature Sir Isaac (1816-1892) Newton. Several honor André- Wb V s Marie Ampère, Allesandro Volta, (1804-1891); no stamp and Nikola . Of the approxi- tesla T Wb/m2 magnetic flux mately 170 stamps recognizing “SI (1856-1943) personalities,” only one has been H Wb/A electric issued by the . France (1797-1878); no stamp and Germany are the most frequent issuers of such stamps. Bq 1/s activity (of a Antoine-Henri radionuclide) Becquerel (1852-1908) SI Base Units Gy J/kg The international system of units (1905-1965); no stamp now recommended for all scientific Sv J/kg dose equivalent Rolf Sievert purposes begins with seven base (1898-1966); no stamp units. Two are named after scien- * The temperature units are the same size. The temperature in Celsius, t, = T – 273.15 K. tists—the ampere and the kelvin. Table I. SI units and the persons for whom they were named.

Metric Units and Postage Stamps Vol. 37, Nov. 1999 THE PHYSICS TEACHER 507 Fig. 3. Fig. 1. Fig. 2.

Fig. 4. Fig. 5. Fig. 6. Fig. 7.

➤The ampere is the SI base unit for knighted by Queen Victoria in 1892) symbol °, unlike the word kelvin and electric current. This unit ties the who introduced the . symbol K for the absolute scale.) electromagnetic and mechanical The one stamp that shows Kelvin The degree Celsius is named after units together into a coherent unit (from Guinea-Bissau, Fig. 2) also Anders Celsius, inventor of the centi- system. The ampere was defined in shows a cable-laying ship, because grade in 1742. How- 1948 as the constant current which, Kelvin was involved in the laying of ever, his original thermometer had if maintained in two straight the first Atlantic cable and became a 100 as the freezing temperature and 0 conductors of infinite , of neg- leading authority in that . as the boiling temperature of . ligible cross section, and placed one The centigrade scale was inverted in meter apart in , would pro- SI Derived Units 1750, a few years after Celsius' death, vide between the conductors a force The remainder of the SI units and his name did not become official- equal to 2 10-7 newton per meter of named after personalities are derived ly attached to the SI unit until almost length. This was an outgrowth of units, units “formed by combining 200 years later, in 1948, making the Andre-Marie Ampère’s law that base or other derived units according term "centigrade" obsolete. The only describes mathematically the mag- to algebraic relations linking the cor- stamp showing Celsius, issued by netic force between two electric cur- responding quantities.” These units (Fig. 3), also shows a ther- rents. Ampère also developed mea- will be considered in order of mometer with the zero point at the suring techniques for electricity. A increasing complexity, with units lower end. stamp from Monaco (Fig. 1) shows defined in terms of other units fol- Ampère and what later became lowing those units used in their ➤The hertz is the derived SI unit for known as a . definitions. frequency . This unit is the reciprocal of the , the SI ➤The kelvin is the SI base unit of ➤The degree Celsius is the derived base unit for . (The symbol for thermodynamic temperature. The SI unit for the ordinary metric tem- hertz is a capital letter H followed by kelvin was defined in 1954 as “the perature scale with values from 0 to a lowercase letter z, to distinguish fraction 1/273.16 of the thermody- 100 degrees between the freezing and this unit from the henry, to be dis- namic temperature of the boiling of pure water cussed later.) The hertz replaces pre- of water,” which is fixed at 273.16 K under standard conditions. The rela- vious usage of cycles per second and (0.01 °C). This definition makes the tion of the degree Celsius scale to the is named after Heinrich Rudolf Hertz. kelvin unit equal in size to a degree kelvin scale is an offset by a constant Hertz helped establish the fact that Celsius, but extending the scale factor of 273.15 K. This is the only SI is electromagnetic . down to . In 1967 the unit with an additive offset, com- The term "Hertzian waves" was once unit name was changed from degree pared with the other derived SI units, popular for what we now call Kelvin to kelvin, dropping the word which are either products or ratios of waves. Representative of the seven and symbol for degree and not capi- two or more SI units. (The word stamps known to honor Hertz is the talizing kelvin. The kelvin is named Celsius and the symbol C are used one from San Marino (Fig. 4), issued after Baron Kelvin (William along with the word degree and the for the centenary of radio in 1995. Thompson, Lord Kelvin when

508 THE PHYSICS TEACHER Vol. 37, Nov. 1999 Metric Units and Postage Stamps Fig. 8. Fig. 11. Fig. 9.

Fig. 10.

Fig. 12. Fig. 13. Fig. 14.

➤The newton is the derived SI unit or power time. His work helped France (Fig. 8) and also pictures a for force, with force being the establish the mechanical equivalent for storing . product of times acceleration. of heat, that heat is a form of Named after Isaac Newton, the unit transfer, and the first law of thermo- ➤The volt is the derived SI unit for reflects Newton's research with the dynamics, or the conservation of electric potential difference or elec- of gravity, leading to his three energy. No stamps are known to show tromotive force. One volt is the dif- laws of motion and his Law of Joule. ference in potential across a conduc- Universal Gravitation. Of the more tor carrying one ampere of current than 80 stamps known to show or ➤The watt is the derived SI unit for when the power dissipated is one honor Newton, the one from power or energy per unit time. watt. Alessandro Volta invented the Germany (Fig. 5) shows another of Although the unit is named after voltaic cell and (or bat- Newton's achievements, his discov- James Watt, he actually coined the tery). One of the stamps from Italy ery in the area of that white nonmetric term "" as a (Fig. 9) issued for the centenary of light is composed of every color in standard for performance of steam radio in 1995 shows Volta and his the spectrum. . A stamp from Wallis and voltaic pile, a feature found on sever- Futuna Islands (Fig. 7) shows Watt al Volta stamps. ➤The pascal is the derived SI unit and a steam and claims Watt for pressure or stress: force per unit as the inventor of the . ➤The farad is the derived SI unit for area. (The symbol for pascal is a cap- Actually, Watt did not invent the electric capacitance, or the ability to ital P followed by a lowercase a.) The steam engine, but made fundamental hold an electric charge. A capacitance unit is named after Blaise Pascal for improvements in its design, resulting of one farad is present when one his work as the founder of hydrody- in a new type of steam engine that coulomb of electric charge causes a namics and the discovery of the basic was patented in 1769 and widely potential of one volt across the plates physical law of pressure, or Pascal's used. of a . This is a very large Law. He worked with atmospheric unit, resulting in most uses as either as well as liquid pres- ➤The coulomb is the derived SI unit microfarads or picofarads for the per- sures. A stamp from France (Fig. 6) for electric charge or quantity of elec- formance of ordinary electric and shows Pascal and two diverse aspects tricity. In terms of SI base units, the electronic equipment. The unit is of his life. The conic sections on the coulomb is the amount of charge named for Michael Faraday, who left reflect his work in geometry; the delivered by one ampere for one sec- discovered the principle of the elec- symbols on the right reflect the reli- ond. Charles Coulomb's Law, which tric motor, based on the dynamome- gious writings of his later life. describes the force between two elec- ter. That principle also laid the foun- trical charges, is similar in form to dation for electrical generators and ➤The joule is the derived SI unit for Newton's Law of Universal . Of the three stamps energy, work, or quantity of heat. Gravitation describing the force known to show Faraday, the one from Named for James Prescott Joule, this between two . The one stamp Great Britain (Fig. 10) is reproduced unit can be either force times distance known to show Coulomb is from here.

Metric Units and Postage Stamps Vol. 37, Nov. 1999 THE PHYSICS TEACHER 509 ➤The ohm is the derived SI unit for ➤The tesla is the derived SI unit for Marie , and France issued the electric resistance. This is the only SI magnetic flux density or webers per stamp shown here (Fig. 14). unit that uses a Greek letter, ⍀, as a meter squared. Nikola Tesla was born symbol. (The only other use of a in Croatia, but spent most of his life ➤The gray and the sievert are the Greek letter in SI terminology is for in the United States. Tesla discovered derived SI units for absorbed dose the micro ␮ prefix, the 10-6 multipli- many of the principles of alternating and dose equivalent, respectively. cation factor.) Georg Simon Ohm's current. A stamp issued by the United Both of these units are measured in Law established the relationship States (Fig. 13) shows Tesla and an per , or energy between the volt and the ampere. The . absorbed per unit mass. These units ohm is the resistance of a circuit in are named after Louis Harold Gray which a potential difference of one ➤The henry is the derived SI unit for and Rolf Sievert. The difference volt produces a current of one electromagnetic inductance or web- between the units is that the first is ampere, or alternately the resistance ers per ampere. Joseph Henry is the the absorbed radiation and the second in which one watt of power is dissi- only American honored by the use of is the equivalent dose of radiation. pated when one ampere flows his name as an SI unit. The principle Both units depend on the of through it. Impedance to alternating of electromagnetic induction was dis- biological effects caused by radia- current and capacitive and inductive covered by Faraday, but named after tion. No stamps are known to show reactance are also measured in . Henry for his independent discovery either Gray or Sievert. The only stamp known to honor Ohm of self-induction. Henry is considered (from Germany, Fig. 11) shows a one of America's great scientists and Comment with colored bands for the was the first Director of the Information given here is verified value of resistance and Ohm's Law . He is also only for those stamps in my posses- with the symbol U for . credited with inventing the first prac- sion. Readers interested in more tical telegraph, which he used to material on stamps honoring those ➤The siemens is the derived SI unit gather weather information. That for whom SI units are named (includ- for electric conductance, which is the effort eventually led to the establish- ing a list of all such postage stamps inverse of the ohm, or one ampere per ment of the U.S. Weather Bureau and souvenir sheets known world- volt. The unit is named after Ernst (later Service). No stamps are known wide, catalog numbers, years of Werner von Siemens, inventor of the to honor Henry. issue, reading lists, etc.) are invited to alternating-current dynamometer or contact me at: motor. Germany honored its country- The last three SI derived units [email protected] or send man on the centenary of Siemens' named after personalities, all with correspondence to 309 North Shores death with the stamp shown in Fig. 12. two-letter symbols, are all related to Circle, Windsor, CO 80550-2614. radioactivity. These units with special ➤The weber is the derived SI unit names were admitted more recently Acknowledgment for magnetic flux or the into SI. They are needed in measure- The author owes the genesis of his of and area. The sym- ments related to interaction of topical stamp collection to the late bol for the weber combines a capital radioactivity and living things. Louis F. Sokol, President Emeritus of letter W followed by a lowercase let- the U.S. Metric Association until his ter b to distinguish the weber from ➤The becquerel is the derived SI unit death in 1996. Many additional the watt. Wilhelm Eduard Weber for activity of ionizing radiation. This stamps were discovered to add to Mr. developed a system for measuring unit is expressed in inverse . Sokol’s original collection, leading to magnetic units. Weber is also noted Antoine-, who discov- this paper. for his study of terrestrial . ered radioactivity, shared the 1903 No stamps are known to show Weber. Nobel Prize in Physics with Pierre and

et cetera... High-Power Lasers: High-Power Density: High Pressure

“Donald Umstadter of the University of Michigan reported on an experiment in which a self-focusing laser beam passing through a reached an intensity of 1020 per square centimeter, the highest yet reported for any laser. In the process of excluding plasma from a narrow region forming a thin channel through the plasma, the laser creates pressures exceeding one giga-bar, et cetera... higher than any other man-made pressure.”1 Column Editor: Albert A. Bartlett, Department of Physics, University of 1. A.P.S. News, July 1996 Colorado, Boulder, CO 80309-0390

510 THE PHYSICS TEACHER Vol. 37, Nov. 1999 Metric Units and Postage Stamps