We illustrate a very curious and interesting form of electric
regulator which is exhibited in the Paris Exhibition of Electricity by
Mr. Killingworth Hedges, whose name will be known to our readers as
the author of a little book on the electric light. Mr. Hedges' lamp
belongs to the same category of electric regulators as the lamp of M.
Rapieff, and to one form of M. Reynier's lamp, that is to say, the
position of the ends of the carbons, and therefore of the arc, is
determined not by clockwork or similar controlling mechanism, but by
the locus of the geometrical intersection of the axes of the carbon
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
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
circuit. The length of the arc can be determined by the adjustment of
the screw, G, by which the amount of movement of the armature is
limited.
Fig. 2 represents a modified form of Mr. Hedges' lamp designed for
installation when it is desirable to burn a number of lamps in series.
In this arrangement the carbons are separated by the attractive
influence of a solenoid upon an iron plunger, to which is attached (by
a non-magnetic connection) the armature of an electro-magnet, the
coils (which are of fine wire) forming a shunt circuit between the two
terminals of the lamp, and so disposed with respect to the armature as
to influence it in an opposite direction to that of the solenoid. When
the circuit of the lamp is completed with the electric generator the
carbons are drawn apart by the action of the solenoid on the plunger,
and the distance to which they are separated is determined by the
difference of attractive force exercised upon the armature by the
solenoid and the magnet; but as the latter forms a short circuit to
that of the arc, it follows that should the resistance of the arc
circuit increase either through the arc becoming too long or through
imperfection in the carbons or contacts, a greater percentage of
current will flow through the magnet coils, and the arc will be
shortened, thereby reducing its resistance and regulating it to the
strength of the current. In other words, the distance between the
carbons, that is to say, the length of the arc, is determined by the
position of the armature of the electro-magnet between its magnets and
the solenoid, which position is in its turn determined by the
difference between the strength of current passing through the coil of
the solenoid and that of the magnet.
Mr. Killingworth Hedges exhibits also a third form of his lamp, in
most respects similar to the lamp figured in Fig. 1, but in which the
ends of the two carbons rest against the side of a small cylinder of
fireclay or other refractory material, which is mounted on a
horizontal axis and can be rotated thereon by a worm and worm-wheel
actuated by an endless cord passing over a grooved pulley. In the lamp
one of the carbon-holders is rigidly fixed to the framing of the
apparatus, and the other is mounted on a point so as to enable the
length of the arc playing over the clay cylinder to be regulated by
the action of an electro-magnet attracting an armature in opposition
to the tension of an adjustable spring.
In the same exhibit will be found specimens of Mr. Hedges' two-way
switches, which have been designed to reduce the tendency to sparking
and consequent destruction which so often accompanies the action of
switches of the ordinary form. The essential characteristic of this
switch, which we illustrate in elevation in Fig. 3 and in plan in Fig.
4, lies first in the circular form of contact-piece shown in Fig. 4,
and next in the fact that the space between the two fixed
contact-pieces is filled up with a block composed of compressed
asbestos, the surface of which is flush with the upper surfaces of the
two contact-pieces. The circular contact-piece attached to the switch
lever can be turned round so as to present a fresh surface when that
which has been in use shows indications of being worn, and a good firm
contact with the fixed contact-pieces is insured by the presence of a
spiral spring shown in the upper figure, and which, owing to an error
in engraving, appears more like a screw than a spring. In order to
prevent bad connection through dust or other impurities collecting
within the joint, the electrical connection between the fulcrum of the
switch lever and the circular contact-piece is made through the bent
spring shown edgeways in Fig. 3.--_Engineering._
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