Study Questions for Bureau of Standards Circular 60

Version of 29 January 2014

• Was this document published before, during, or after World War I?
• The earliest (mid-nineteenth century) widely-used electric units measured resistance (for example, page 18). Why resistance?
• Who prominently urged adoption of the cgs system of units, and when?
• Whose use of the millimeter-milligram-second system of units led others to adopt it? When and where?
• On page 4 the author distingushes three types of standards. Which of his types would the standards defined by the 1893 Chicago conference fall into?
• On page 5 the author gives an interpretation of the meaning of “standard” when used for derived units. Is it possible for the magnitude of a derived unit, as determined from its standard, to differ from the unit's magnitude as determined from the “primary standards” of the units used to define the derived unit? Justify your answer.
• How could the B. A. unit be “in error by more than 1 per cent”? (page 19)
• On page 16 the author describes the silver voltameter as the “primary standard of current.” Is such a standard defined in terms of fundamental dimensions, like length, mass and time? In what ways is his usage on page 16 inconsistent with his earlier (page 4) definition of primary standard? Or is it?
• The Edinburgh meeting of 1892 and the Chicago conference the following year specified experimental methods for determining the values of the ampere, ohm and volt “sufficiently well for practical use”. Why was this ultimately an unworkable idea? The attendees at the conferences weren't dumb. They certainly knew I = E over R. Why then did they set standards for all three units?
• In Table 1, the author gives lengths in centimeters rather than meters. Why?
• Write an example illustrating the usefulness of dimensional expressions in checking equations. Test any simple equation from physics, but do not use one of the author's examples.
• Can two units have the same dimensions and yet describe different properties? If your answer is yes, give an example.
• The author implies (page 16) a preference for choosing the volt as the second electric unit to be internationally standardized, instead of the ampere. He says that was the position of the U.S. Bureau of Standards (page 25). The international conferences, however, chose the ampere.
  Why did the author prefer the volt?
  Why did the conference vote for the ampere? (Consider page 16.)
• Examine the resolutions of the 1893 Chicago conference (page 22). Why do the words “represented by” appear in most of the definitions? Why do they not appear in the definitions of the farad and coulomb?
• The author speaks of the “ether” (pages 14 and 15) What was the ether?
  This publication appeared in the same year that Albert Einstein published the General Theory of Relativity, 11 years after the Special Theory of Relativity, and 29 years after the Michelson-Morley experiment. Michelson himself apparently never gave up on the concept of the “luminiferous ether.” Do you think the author's use of the concept is justified? Why or why not?
• The author suggests (page 5 —and he wasn't the first) that the unit of length might be defined in wavelengths of light instead of a particular metal bar. In 1960 the 11th Conférence Générale des Poids et Mesures (Resolution 6) in fact replaced the International Prototype of the Meter with “1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom.” What took so long?
• The author mentions a number of problems in the systems of electric units in use at that time, many concerning the minute differences between the units used in scientific research and those used in technology. As time passed and the precision of measurements improved, these discrepancies became more onerous (compare page 21), and would eventually lead to the abandonment of the international units and even the cgs system itself, and the adoption of the coherent system we now use. What were some of the problems?