Response to Comments on "SI Units? A Camel is a Camel" I am mateful t o the editor of the Journal of Chemical Education for t o resoond t o the oaoers hv Nelson and hv ..-thennnortunitv .~ .rr.-.~ . ~~~~, Wright, and to the Letters from ~ e s l i and p frbm Dingledy regarding my article on SI units (J.CHEM. EDUC., 55,634 (1978)). The opportunity should he used primarily for clarification of position, but a minor riposte or two is surely allowable as well. Nelson's point about the distinction between a unit and a standard is an imoortant one. It hears on my position regarding Coulomb's law. & have no standard of ch&& or of force, in the wnsc cr :r itandnrd mrter nd or n standard Pt-lr rylmder. Rather, w r rlef.nc tom? nnrl c h a r p in termsof m appmpriaw fundamental law uinaturr. llnit torcerould he definrd tv means ofthe gmvitational law, which would then he written ~~
~~~
~~
.
~~~~~
~~
~
F = -m1mzlr2
(1)
the gravitar,onal constnnt G t h w disnppcariw In a differrnr world, on a dlffrrenr planet, this might he mnwnient. For us, the ~ Nem0n.s law. greater conwnience and relevance is i n t h useof F = ma, to define force. We accept the consequence of having to introduce aconstaut G ineqn. (1); in chemistry, a t least, the gravitational law is not a central one. There is again a choice in the case of charge. Unit charge may
, ~ ~ . .~
he defined in terms of Coulomb's law (eon. (1) ~. .. . in mv orieinal "~ ~~~
~~
~
paper), as is done in the conventional system of units. Alternatively, it may he defined in terms of a law of electromagnetism, as is done in the S I system (uia the definition of Ampere) Thus quantities not representable by a physical standard are defined operationally by means of a Law of nature, and since usually more than one law may he invoked, the choice becomes one of convenience and of relevance. The m i n t I made is that the relevant choice in cherniqtrv is that of definine charm in terms of ionic interactions is thus emphasized and the intrusion of the "Finagle constant" 4mo does not occur. In the conventional system of units, the convenience of the electromagnetic definition of current is al& recoenized. hence the concurrent existence of emu and pracricol uni&;fch&e and uf potrntial. The SI method, in demanding a single tunic of charge, arhieves uniiurmity, but dues so nt the expense of making Cc,uloml,'s lau second class. By second class, I mean a law that is not regarded as central enough to be used as a defining law for a physical quantity. Since the law is not in fact a second class law to chemists, the result is the intrusion of 4aro factors throughout physical chemistry.
Volume 56, Number 10, October 1979 1 665
