This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
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.
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.
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.