rods, the positions of which axes being determined by simple
mechanical means.

[Illustration: Figs. 1 and 2 HEDGES' ELECTRICAL LAMP AT THE PARIS
ELECTRICAL EXHIBITION.]

Referring to Fig. 1, A and B are two troughs rectangular in cross
section attached to the supports in such positions that their axes are
inclined to one another so as to form the letter V, as shown in the
figure. Within these troughs slide freely the two carbon pencils,
which are of circular cross section, meeting, when no current is
passing, at the lower point, E. The carbon-holder, B, to the right of
the figure, is rigidly attached to the framing of the lamp, but the
trough, A, which carries the negative carbon, is attached to the
framing by a pivot shown in the figure, and on this pivot the carbon
holder can rock, its motion being controlled by the position of the
armature of an electro-magnet, M, the coils of which are included in
the circuit of the apparatus. By this means, the moment the current is
established through the lamp, the armature is attracted, and the
points of the two carbons are separated, thus forming the arc. The
positive carbon, B, is held from sliding and dropping through the
trough by the gentle pressure against it of the smaller carbon rod,
C¹, which also slides in a trough or tube fixed in such a position
that the point of contact between the two rods is sufficiently near
the arc for the smaller rod to be slowly consumed as the other is
burnt away; the latter in that way is permitted to slide gradually
down the trough as long as the lamp is in action. The negative
carbon-holder, A, is provided with a little adjustable platinum stop,
E, which by pressing against the side of the conical end of the
negative carbon, holds the latter in its place and prevents it sliding