For ten points, name the value from which the SI unit for distance is derived.
This value, represented by a capital V in the 19th century, later changed symbols after an 1856 paper by Wilhelm Weber. Planck and Lorentz adopted the new symbol, and Einstein switched his notation accordingly. Asimov referred to it by the Latin word for 'swiftness', though Weber simply meant for it to stand for 'constant'. For ten points, name the value from which the SI unit for distance is derived.
This value, represented by a capital V in the 19th century, later changed symbols after an 1856 paper by Wilhelm Weber.
{speed of light} (prompt on "c")