r.m.s. [root-mean-square]
electromagnetics The statements of simple relationship between volt, ampere, ohm, and other electromagnetic units (e.g. resistance in ohms = V·A-1, electric charge = A·s) apply basically only to direct current. For alternating current, with its inherent oscillation of voltage between some matched positive and negative figures of maximum amplitude, the same statements can apply if an average is used - an average not of the voltage itself, but relative to its effective power level - to give a figure for voltage equal to that for a steady direct current with the same heating effect. Allowing that power is proportional to the square of voltage, that average is the square root of the mean of the voltage squared. Termed root-mean-square or simply r.m.s., for a smooth sinusoidal oscillation it equals the maximum amplitude divided by
; identically the maximum amplitude =
·r.m.s. The stated voltage on systems and equipment is usually the r.m.s., so the 110 V a.c. of North America oscillates between ±110·
V≈±156 V and the 230 V a.c. of Europe oscillates between ±230·
V≈ ±325 V.
The concept of r.m.s. value is applied similarly to sound and other oscillatory phenomena.
