
73
Generalising this calculation, we find that when an alternating
voltage is adjusted to deliver the same power to a given resistance as a
direct voltage, its peak value is ^2 times the direct voltage, or about
1.414 times as great. Put the other way round, its nominal or
equivalent or effective voltage—called its r.m.s. (root-mean-square)
value—is 1/^2 or about 0.707 times its peak value. Since the
resistance is the same in both cases, the same ratio exists between
r.m.s.
and peak values of the current. What is called a 240-V a.c.
supply therefore alternates between + and
—
^2 χ 240 = 339 V peak.
Since the r.m.s. values of voltage and curren ...