These two
torques
must
be
equal
in
equilibrium.
Hence,
MH
sin
u
=
Mi
^
cos
u
R
or
i
=
H
tg (u)
.
If
H
is known in absolute
units, then thereby the absolute unit of i
is also defined.
One
can
posit:
i
=
1 when the
righthand
side
of the
expression
=
1.
The Paris
Congress established
an
additional,
socalled
practical
unit,
which is 10 times
smaller.
The
relation
then becomes
i
=
10H
tg
u.
2tr
R
is called the constant of the
galvanometer.
In
our
region
H
equals
about 0.21 in
g
cm
sec.
The absolute
unit of current has been
introduced by Wilhelm Weber.
The
reading
method
of Gauss and
Poggendorf.
One gets
tg (2u)
=
aD.
[Fig.]
Were
one
e.g.,
to
choose
D
=
2m,
then
a
rotation of 1° would
yield
tg 2u
=
0.0348
tg 2u.D
=
70
mm.
If
readings
can
be made with
an
accuracy
of

mm,
then it follows
that
an
accuracy
of
^0°
is
attainable for this distance.
Intervals in which currents
can
be measured in this
way
are
relatively few.
To be able to
measure
small currents,
one
only
needs
to
multiply
their number of
windings.
Each
winding
then
exerts
an
almost
equal
effect
on
the
ring
magnet
so
that
one
obtains
R
i
=
H
tg
u
27T n
where
R
denotes the
average
radius of the
windings.
121