Note 1 — Chapter XLVI · ALTERNATING CURRENTS

NOTE.—"I adhere to the term virtual, as it was in use before the term efficace which was recommended in 1889 by the Paris Congress to denote the square root of mean square value. The corresponding English adjective is efficacious; but some engineers mistranslate it with the word effective. I adhere to the term virtual mainly because the adjective effective is required in its usual meaning in kinematics to represent the resolved part of a force which acts obliquely to the line of motion, the effective force being the whole force multiplied by the cosine of the angle at which it acts with respect to the direction of motion. Some authors use the expression 'R.M.S. value' (meaning 'root mean square') to denote the virtual or quadratic mean value."—S. P. Thompson.