
18-1
Instruments for the measurement of electric voltage are called voltmeters. Correct insertion of a voltme-
ter
re
quires
th
e
co
nnection
of it
s
te
rminals
to th
e
po
ints
of an el
ectric
ci
rcuit
ac
ross
wh
ich
th
e
vo
ltage
ha
s
to be me
asured,
as sh
own
in Fi
gure
18.1. To a r
st
ap
proximation,
th
e
el
ectric
eq
uivalent
ci
rcuit
of a
vo
ltmeter
ca
n
be re
presented
by re
sistive
im
pedance
Z
v
(or a pure resistance R
v
for dc voltmeters). is
me
ans
t
hat
a
ny
v
oltmeter,
o
nce
c
onnected
to a
n
e
lectric
c
ircuit,
d
raws
a c
urrent
I
v
given by
I
V
Z
v
v
=
(18.1)
where V is the measured voltage. e higher the value of the internal impedance, the higher the quality
of t
he
v
oltme ...