ELECTRIC LIGHTING. In 1800 Sir Humphry Davy discovered that if two pieces of carbon are joined by conductors to a source of electric current, and such pieces momentarily touched together, so that the circuit is complete and a flow of current established, upon their separation for a short distance, a flame is emit ted, and, if the current be sufficiently powerful, this flame will continue, the carbon points will become intensely hot and emit a brilliant light. In separating the carbon points, the extra po tential induced by the self-induction of the cir cuit is sufficient to leap the small air gap and thereby vaporizes a small quantity of carbon. Carbon vapor, being a conductor, allows the current to flow across the gap. The vapor is of high resistance, and therefore it is heated to a high temperature. In 1809, Davy exhibited his arc light before the Royal Institution of London, his carbon points being charcoal from the willow, and his source of current a voltaic pile.
We do not know in which direction an elec tric current flows through a circuit, and there are many reasons for believing that it flows both ways at the same time.• It is usual, how ever, to assume that it flows in one direction only, namely — from what is called the *posi tive pole of the generator, through the exter nal circuit to what is called the *negative pole.* According to this view, when a direct, continu ous current flows between two slightly separated carbons so as to form an arc, the electricity tears away particles of carbon from the positive electrode, and deposits some of them upon the negative one. Some of the particles become burned in the passage, so that both carbons waste away; but the positive carbon wastes the faster and becomes hollowed out, while the negative one wastes less rapidly and assumes a pointed form. The temperature is high enough to melt the most infusible substances, such as flint, platinum and the diamond. The carbon points emit the larger portion of the light, and the positive point more than the negative. The resistance of the arc may vary from 1 to 100 ohms. It requires a current strength of from 3 to 10 amperes, and a minimum electromotive force of 40 to 50 volts, to maintain a satisfac tory lighting arc.
Davy used wood charcoal for his electrodes, and while they were of excellent quality, on account of their softness they lasted only a short time. As the arc would burn away, it was necessary continually to adjust them, or the arc would go out. Therefore it was found nec
essary to have carbons of sufficient density to last a reasonable time, and purity so that the light might be steady. Also to have a mechan ism automatically to feed the carbons, and keep them a constant distance apart, as well as auto matically bring them together in order to start the arc. Refined petroleum coke, ordinary gas coke, or lamp black is now taken for the base material, thoroughly ground and mixed into a stiff paste, dried and carbonized out of contact with the air. A very hard and uniform carbon is thus obtained, in any desired size, the usual length being 12 or 14 inches, and diameter 7/10, or inch. Broadly, the lamps .may be divided into two classes: series and multiple; each system into two sub-divisions: the open and the enclosed.
The electric energy loss from the point of ge'neration to the lamp may be expressed, W - CR; where W is energy in watts, dissipated as heat in conductors, C is current in amperes, and R is resistance of circuit in ohms.
From the equation it may be noted that the loss is in proportion to the square of the cur rent so long as R remains constant. Therefore, in distributing electric energy to a number of arc lamps, it is more economical to keep the cur rent constant, and have the lamps joined in series.
Two types of lamps were evolved to meet these conditions as well as a generator to keep the current at a constant quantity, (1) the dif ferential lamp; (2) the shunt lamp. In the differential lamp, the current must retnain at a constant value. The main current is carried to a pair of lifting magnets, over which, but wound in opposition thereto, is a coil of high resistance called the shunt magnet, the terminals of which are cut around the arc itself. When current is thrown into the lamps, the main cur rent pulls the carbons apart until sufficient potential is obtained at the arc to force current through the shunt magnet, which neutralizes the main magnet, thereby securing a balance, and maintaining the potential at the arc constant. In the shunt type lamp, the shunt magnet cir cuit is so arranged as to trip the clutch holding the carbon rod, thereby allowing the carbons to come together, should the potential around the arc exceed the predetermined amount This lamp will burn upon any current strength, pro viding it is enough to operate the main magnet.