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

 T      he metric system has evolved
        from the French (“metre”)
 units of 1790 to be officially recog-
                                           SI Unit


                                                                       Compound Quantity
                                                                       base unit electric current

                                                                                                                 André-Marie Ampère
 nized by the Eleventh General
                                           kelvin            K          base unit      thermodynamic             Lord Kelvin/William
 Conference on Weights and                                                             temperature               Thomson (1824-1907)
 Measures as the International
                                           degree            °C         K*             Celsius                   Anders Celsius
 System of Units (SI) in 1960. This
                                           Celsius                                     temperature               (1701-1744)
 note brings together brief biograph-
                                           hertz             Hz         1/s            frequency                 Heinrich Rudolf Hertz
 ical notes of the scientists for whom
 many SI units are named, along
 with reproductions of postage             newton            N          kg m/s2        force                     Isaac Newton
 stamps from around the world that
 have been issued to honor these           pascal            Pa         N/m2           pressure, stress          Blaise Pascal
 men. This combination of science,                                                                               (1623-1662)
 metrics, and history may also pro-        joule             J          Nm             energy, work,             James Prescott Joule
 duce the bonus of introducing                                                         quantity of heat          (1818-1889); no stamp
 someone to the hobby of topical           watt              W          J/s            power, radiant flux      James Watt
 stamp collecting.                                                                                              (1736-1819)
    Currently, 19 SI units are named       coulomb           C          As             electric charge,        Charles Coulomb
 after scientists (see Table I). Each SI                                               quantity of electricity (1736-1806)
 symbol is unique in its spelling          volt              V          W/A            electric potential       Alessandro Volta
 (usually a capital first letter of the                                                difference,              (1745-1827)
 person’s name), and each unit is                                                      electromotive force
 unique scientifically, describing a       farad             F          C/V            electric capacitance     Michael Faraday
 quantity to be measured using that                                                                             (1791-1867)
 SI unit. (Celsius alone is                ohm                          V/A            electric resistance      Georg Simon Ohm
 capitalized.)                                                                                                  (1787-1854)
    By far the largest number of SI        siemens           S          A/V            electric conductance Werner von Siemens
 postage stamps feature Sir Isaac                                                                           (1816-1892)
 Newton. Several honor André-              weber             Wb         Vs             magnetic flux            Wilhelm Eduard Weber
 Marie Ampère, Allesandro Volta,                                                                                (1804-1891); no stamp
 and Nikola Tesla. Of the approxi-
                                           tesla             T          Wb/m2          magnetic flux density Nikola Tesla
 mately 170 stamps recognizing “SI                                                                           (1856-1943)
 personalities,” only one has been
                                           henry             H          Wb/A           electric inductance      Joseph Henry
 issued by the United States. France
                                                                                                                (1797-1878); no stamp
 and Germany are the most frequent
                                           becquerel         Bq         1/s            activity (of a           Antoine-Henri
 issuers of such stamps.
                                                                                       radionuclide)            Becquerel (1852-1908)
 SI Base Units                             gray              Gy         J/kg           absorbed dose            Louis Harold Gray
                                                                                                                (1905-1965); no stamp
     The international system of units
 now recommended for all scientific        sievert           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 absolute scale.        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 thermometer 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 parallel              the first Atlantic cable and became a     100 as the freezing temperature and 0
conductors of infinite length, of neg-              leading authority in that field.          as the boiling temperature of water.
ligible cross section, and placed one                                                         The centigrade scale was inverted in
meter apart in vacuum, 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 x 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       Sweden (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 galvanometer.                            definitions.                              frequency measurement. This unit is
                                                                                              the reciprocal of the second, the SI
➤The kelvin is the SI base unit of                  ➤The degree Celsius is the derived        base unit for time. (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 triple point               boiling temperatures 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   light is electromagnetic radiation.
down to absolute zero. 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 radio
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 times time. His work helped          France (Fig. 8) and also pictures a
for force, with force being the              establish the mechanical equivalent           Leyden jar for storing electric charge.
product of mass times acceleration.          of heat, that heat is a form of energy
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-
forces 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 voltaic pile (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 optics that white         nonmetric term "horsepower" 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.                                engines. 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 engine and claims Watt
for pressure or stress: force per unit       as the inventor of the steam engine.          ➤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 capacitor. This is a very large
Law. He worked with atmospheric                                                            unit, resulting in most uses as either
gas pressures 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                     transformers. 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 masses. 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 Curie, 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        joules per kilogram, or energy
between the volt and the ampere. The            induction motor.                           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 nature 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 ohms.            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
resistor with colored bands for the             was the first Director of the                 Information given here is verified
value of resistance and Ohm's Law               Smithsonian Institution. He is also        only for those stamps in my posses-
with the symbol U for voltage.                  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 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 dot product            into SI. They are needed in measure-          The author owes the genesis of his
of magnetic field 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 seconds.      Sokol’s original collection, leading to
magnetic units. Weber is also noted             Antoine-Henri Becquerel, who discov-       this paper.
for his study of terrestrial magnetism.         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 plasma reached an intensity of 1020 watts per square centimeter, the highest yet
       reported for any laser. In the process of excluding plasma electrons 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