- Is the SI the same as the metric system?
The SI is the modern form of the metric system.
The metric system was originally established after the French Revolution as a
consistent set of decimal units with base units that could be precisely
determined by reference to standards. The SI (officially established in 1960)
differs from older versions of the metric system in the number of base units and in the way these base units are
defined. For example, the older system was based on centimeters, grams, and
seconds; the modern SI is based on meters, kilograms, seconds, kelvins, moles,
amperes, and candelas.
- Is it "kelvins" or "degrees Kelvin"? Celsius? Centigrade?
It's "kelvins" (small k) with the symbol K (capital K).
In older material you'll run across "degrees Kelvin," symbol °K, but that usage was officially declared obsolete in 1980.
Celsius temperature is expressed in degrees Celsius (small "d", capital "C"), with symbol °C. The old term "degree centigrade"
was officially eliminated in 1948.
- What about the decimal point? I've seen a comma instead of a period.
In English - both American and British - the decimal marker is the point on the line: 3.14,
with at least one digit before the decimal point: 0.25 s rather than .25 s. In reality, either the point or the comma is correct.
- Does the non-metric U.S. system of measurements have a name?
There's no official name for the non-metric U.S. system of weights and measures, but the most commonly used terms are:
"customary system" or "customary units" or "customary weights and measures" or "inch-pound units".