Letters on SI Units To the Editor: As co-author of one of the books mentioned in "SI units? A Camel is a Camel," I should like the opportunity to comment. The author launches a strong attack on SI without understanding the system very well. The fact that he uses "The SI system" as one of his crossheadings is itself a clear indication that he has not devoted much thought to the matter. As SI means SystAme Internationale, does he really need it as a n adjective to qualify the noun system? Of the many points which I could raise I should like t o choose one. The ereat difficultv for students in havine an electrostatic unit of current and also an electromagnetic unit is that t h w are different in dimension. The former unit has X (time)F2 whereas the d i m e n s i ~ n ( l e n g t h ) ~X/ ~ X (mass)'/2 X (time)-1. The latter has dimension (leneth)"2 . modificatiun oi the Coulomh and Law equations war made neressarv bv this fact. nut bv the introdurtion of SI. What the sf dies is to replace many units, of current for example, by one unit. Oneis surely enough forany onephysical quantity! In defense of Europeans, we find it mildly amusing that the countrymen of Mark Twain and Ogden Nash should have sponsored a unit called the U.S. international ampere and not recognized it as funny. We can, and do willingly, accept Jack Nicklaus as a US. international golfer; the same description applied t o a unit does not seem to make much sense.
ere
To the Editor: Arthur Adamson in the article on S I units (J. CHEM. EDUC., 55,634 (1978)) chooses a poor example to illustrate the "direct and natural" use of the cgs system: force
=%El? r2
but instead points out the inherent difficulties. There are nine units for current, charge, and potential in the cgs system, related in a somewhat complex way, and the required electrostatic charge unit is 0.33 X 10-9 coulombs. If the corresponding SI expression force =
1 9'42 41ro
r2
is used, the fundamental electrical charge unit, the coulomb, is used. The permittivity constant, €0, used in the SI expression, is not a mysterious "Finagle constant," (as described by Dr. Adamson), but i t is determined by experimental measurements of capacitance in a capacitor of known dimensions where
c =d -. d
where C = capacitance, A = plate area, d = distance between plates. David Dingledy State University College Fredonia, NY 14063
The University of Manchester Institute of Science and Technology P.O. Box 88
Manehester M60 IQD, England
VOlum.? 56, Number 10. October 1979 1 665
