Donald's summary was nearly exhaustive, but I have one more unanswered
question about form when discussing physical quantities. When a physical
quantity stands in the role of an adjectival phrase, should one intercalate
a hyphen between the number and its units? E.g. "I wouldn't touch it with a
ten-foot pole." "He finished last in the 100-m dash." "A 55-kg ballet
dancer is spinning on point at 1.4 revolutions per second."
It is my experience that standard physics textbook style almost always
includes the hyphen, but I prefer to leave hyphens out if they are not
needed to resolve ambiguity. Fowler's entry on hyphens pretty much comes
down on the same side, though it opens with the sentence "No attempt will
be made here to describe modern English usage in the matter of hyphens; its
infinite variety defies description."
By the way, I still use mhos (with the inverted omega symbol) and I want to
see some reference with a transconductance expressed in sienmenses (I
assume that is the correct plural). The committee that decided on that unit
eveidently did so on political grounds and without regard to tradition and
aesthetics. Another SI unit I won't buy is the becquerel (not to be
mistaken for the element beryllium). I will teach a bunch of biology
students (who will be using radioisotopes) about radioactivity in a couple
of weeks. I would be utterly derelict were I not to teach them that
1 Ci = 3.7E10 d/s = the decay rate of a gram of radium. The Roentgen*-rad-
rem-(rbe)-gray problem is even knottier, but I'll certainly try my best to
raise their collective consciousness about it.
Leigh
*The Roentgen (or with unmlaut) never was an SI unit, and I've never seen
it in lower case. Unfortunately the same symbols (R or r) seem to be used
interchangeably for different units! This is a classic case in which SI
has brought order to a previously chaotic area. Conventions can be good!