down the trough except under the influence of the slow combustion of
the cone during the process of producing the arc. The position of the
stop with respect to the conical end is determined by a small
adjusting screw shown in the figure. This arrangement of stop is
identical in principle with that adopted by Messrs. Siemens Brothers
in their "abutment pole" lamp, and is found to work very well in
practice on the negative electrodes, but is inapplicable on the
positive carbons on account of the higher temperature of the latter,
which is liable to destroy the metallic stop by fusion, and it is for
this reason that the positive carbon in Mr. Hedges' lamp is controlled
by the method we have already described. For alternating currents,
however, the abutment stop may be used on both electrodes.

[Illustration: Figs. 3 and 4.]

In order to maintain a good electrical contact between the fixed
conducting portions of the lamp and the sliding carbons, Mr. Hedges
fits to each carbon-holder a little contact piece, F F, hinged to its
respective trough at its upper end, and carrying at its lower or free
end a somewhat heavy little block of brass grooved out to fit the
cylindrical side of the carbon, against which it presses with an even
pressure. This arrangement offers another advantage, namely, that the
length of that portion of the carbon rods which is conveying the
current is always the same notwithstanding the shortening of their
total length by combustion; the resistance of the carbon electrodes
is, therefore, maintained constant, and, for the reason that the
contact piece presses against the rods very near their lower ends,
that resistance is reduced to a minimum. In this way very long
carbons, such, for instance, as will burn for ten or sixteen hours,
can be used without introducing any increase of resistance into the