182
DOC.
14 PROOF
OF
AMPERE'S
CURRENTS
705
mportance.
The double deviation
remained
unchanged
when the
position
of the
axis
of the
coil
with
respect
to
a
vertical line
was
changed
by means
of the
foot
screws, a change
which
gave
rise
to
horizontal
alternating
fields.
Effect
III, however,
which
was
caused
by
the action which stati-
onary
magnetic
fields
can
exert
on
the
alternating
poles
on
account
of
their excentric
position
could
easily
be
observed. The double
deviation
changed immediately
when
a
permanent magnet
was
brought
near
the
coil. The influence
of the terrestrial
magnetism
was
also
apparent.
When it
was
not
compensated we
got,
in
the
case
of
resonance,
a
broadening
of
the
image
on
the
scale
up
to
3
cm
for
a
scale
distance of 45
cm.
In all further
experiments
the terrestrial
field
has therefore been
compensated,
the
measure-
ments
required for
this
being
made
with
an
earth inductor and
a
ballistic galvanometer.
The horizontal and vertical
components
of
the terrestrial
field
were
compensated separately
by means
of
hoops
of about
1
m.
diameter
on
which
copper
wire
was
wound. The
current
was
taken from
storage cells,
and
precision
Amperemeters
of
Siemens
and Halske served for
continually controlling
its
strength.
Whether
the
compensation
was
obtained
could be tested
by
turning
the
upper
end
of the
suspending
wire. The
amplitude
of the
oscil-
lations
changed
by
this
so long as
the terrestrial
magnetism
was
still
acting
on
the iron
magnetized
by
the
alternating
current.
After
compensation
however
this
azimuthal
sensibility
of the
effect had
disappeared.
After all there remained
a
well marked double devi-
ation of
4,5
mm.
We
now
had
to
make
sure
that this
was really
the
effect
we
sought for.
For
this
purpose
we
first
availed ourselves of the cir-
cumstance that the
acting
couple
must differ
a
quarter
of
a
period
in
phase
from
the
current
and the
magnetisation.
We
brought
a
permanent magnet
near
the
coil,
thereby calling
forth
effect
III
and
adding
to
the
couple
B1
cos
wt,
with which
we are
concerned,
a new
one,
which has
the
same or
the
opposite phase
as
the
magne-
tisation
and therefore
differs
a
quarter
of
a period
in
phase
from
B1
cos
iot.
Whatever
be
the
sign
of
this
additional
couple,
the
ampli-
tude of the
resulting one
must
become
larger
than
B1.
We
found
indeed
that the
broadening
of the
image always
increased when
we
brought
a
magnet near
the
coil.
Further
the
theory requires
that the
magnitude
of
the effect
depends
on
the
intensity
of the
alternating
field
in the
same way
as
the
magnetisation
itself. This
was
likewise confirmed
by
experiment.
Finally we
shall
compare
the observed
magnitude
of the
effect