
25
had fallen to 900x0.007 = 6.3 V. Note that in driving current
through a resistance an
e.m.f.
causes a p.d. of the same voltage
between the ends of the resistance. This point is elaborated in Sec.
2.14.
2.7 Larger and Smaller Units
It is unusual to describe a current as 0.007 ampere, as was done just
now; one speaks of '7 milliamperes', or, more familiarly, '7
milliamps'. A milliampere is one-thousandth part of an ampere.
Several other prefixes are used; they are tabulated in Appendix D,
the commonest being
Prefix
Meaning
Symbol
milli-
one thousandth of
m
micro-
one millionth of
μ
nano-
one thousandth-millionth of
η
pico
one billionth of (millio ...