VOLTAIC ARC LAMPS. coil is at A,
magnetic
I
79
above the upper carbon-holder, and
is
supported by the arm b fixed to a vertical piece c sliding within a column so as to adapt the apparatus to different Inside of the coil a magnetic core
lengths of carbons.
moves
freely.
hollow, and
is
d
This core through
passes the copper rod // of the upper carbonit
holder, sliding freely for
A sort
whole length.
its
of
h
collar
grasps
it,
however, a little below the magnetic core, and is
so arranged that
it
on
presses
a
when cross
piece //, forming .part of the fixed system of the
arm
//
leaves the rod
b> it
free,
which then de-
scends by
The
own weight.
its
collar
therefore
maintains the rod only
when up,
it
and
is
itself raised
this takes
place
almost constantly during the working of the apparatus, for a catch e fixed
on the supports in
its
magnetic it
rise
core
above; but it cannot go
FIG. 46.
beyond a certain limit which can be adjusted, for a screw .v has a head large enough to hold it by its upper edge. The magnetic core itself is kept up by a cross piece on which act two spiral springs, the rods of which serve at the same time as particular feature
;
guides.
one
is
The
carbon-holders have no
fixed to rod
//
the other to an 12
2