This number
applies
for 0°.
For the
temperature t
we
have
w
=
w0(l
+
0.0041t).
Wi5
- 7660
(approximately).
Measurement of the current.
A
direct
measurement of the current,
i.e.,
of the
quantity
of
electricity passing
through
the
cross
section of the conductor in unit
time,
is not feasible. The current must therefore be determined from
its
effects. For that
purpose, one uses
almost exclusively the
magnetic
effect.
Again
we
start from
a
hypothetical
fundamental
law and
compare
the
consequences
with
experience.
If
a
magnetic
mass
is
brought
near a
part of
an
electric
circuit,
a
force will
be exerted
upon
it
perpendicular
to the direction of the
current element
&
l
to the direction of the connection between the
mass
and the current element.
M
[Fig.]
north
The
direction
of this force is obtained in the
following
way.
An
observer swimming with the
current
with his head forward
sees
the
force exerted
on
the north
magnetic
mass as
directed to the left and
that exerted
on
the south
magnetic
mass
in
the
opposite
direction.
In accordance with the
principle
of the
equality
of action and
reaction, the magnetic
mass
exerts
a
force of the
opposite
direction
on
the current element
.
This is demonstrated
by the arrangement (Ampere) shown in the
accompanying
scheme.
[Fig.]
The
elementary
law of the electromotor
magnetic
force
was
established by Laplace
on
the basis of Biot's and Savart's
investigation
on
the
magnetic
effect of long linear currents.
It reads
r[Fig.]
Tl •
1
i
j-
d
S
171
,
df
=
-^-2
sin
r
.
The direction is
as
indicated above.
A
direct
proof for
this
law is not
feasible
but the conclusions
deduced from it
are
in
complete agreement
with
experience.
117