The conditions common to both types of series lamps are: (1) As the carbons burn away, they must be fed down gradually and not allow the potential around the arc to exceed a certain voltage. (2) Should the carbon rod stick, or anything happen to disarrange the lamp, protective devices called cut-outs come into operation, so the lamp will not be burned up, or the circuit opened. These open types of lamps are now practically obsolete.
'The Enclosed The open arc was superseded by what is called the enclosed arc. It was discovered that if the arc be surrounded by a narrow, nearly air-tight globe, it greatly modified the character of the light and the car bon would last much longer. The small globe prevents the air from having access to the car bon, thus diminishing its oxidation. When the lamp is started, it soon exhausts the oxygen contained in the globe, and if the globe is tight, the carbons will last from 80 to 175 hours. This type of lamp held the market for some years and then the flame arc lamp became popu lar.. This. was. based on the principle of intro ducing oxides into the carbons, to secure an in creased flame arc, somewhat as the Welsbach mantles increase the luminosity of a gas-flame. Calcium chloride, and the oxides of iron, titan ium, chromium, etc., were employed with more or less success. In the making of the car bons (which' are commonly of petroleum coke ground fine and mixed with tar, forced through dies and dried), the metallic oxides are either mixed with the mass or introduced in a metal lic core. An increased voltage is required, as the electrodes are drawn farther apart to get a long flame; in some forms, as the magnetite lamp, the lower positive electrode is simply a permanent knob of copper, while the upper negative electrode carries the mineral salts. Such lamps give a large body of arc or flame and do not burn a crater in the carbon, but consume a comparatively flat surface when di rectly opposed. In one form two carbons are employed, positioned like the sides of a V. The quantity of vapor given off by the flame arc lamps necessitated an open globe at first, but later it was found possible to draw out the vapor by a draft and largely to enclose them. • Glowers and Vapor The Nernst lamp was the next popular favorite. Instead of carbons it employs what is known as a glower, this being a little tube, something over an inch long, and made of the oxides of tho rium, zirconium, yttrium, etc., mixed with a binding paste and baked into a condition re sembling porcelain. This tube is coated with oxides and provided with platinum terminals. On passing a current it emits a soft white light The glower has to be heated to start it, and a coil called a heater is provided for the purpose; there is also a °ballast* or resistance to avoid overheating. After the lamp glows it provides
its own heat and the coil is automatically switched out. These glowers are of about 50 candle power and when mare light is demanded, several glowers are included in one lamp. The life of a glower is about 600 hours on direct circuit and 400 on alternating current, thus far outtlassing the arc lamps. They are made for both 110 and 220 volt circuits.
A distinct advance in durability and long life of the worlcing parts of an electric lamp has been scored by the so-called vacuum tube lamps, in which a long tube is exhausted of air and then supplied with a small quantity of me tallic vapor, as of mercury, which lights up when an electric current is passed through, emitting a very soft diffused light, that does not trouble the eyes with its glare, as is the case with most strong lights. The Cooper-Hewitt mercury vapor lamp was first tried out in 1903, at the office of the Evening Post, in New York, and has since been largely introduced for gen eral indoor use. The typical house-lamp em ploys a tube from 20 to 50 inches long and of perhaps an inch diameter, backed by a metallic reflector. At one end of the tube is an iron mercury cup serving as the positive electrode, while a glass bulb at the other end is the nega tive terminal. Platinum wires supply the cur rent The air is exhausted from the tube, which is then sealed. When the current flows, a little tnercury is allowed to vaporize in the tube and a soft greenish-white light of great intensity is obtained. Induction coils are placed in the body of the fixture, also an adjuster resistance, shunt resistance and cut-out. Pulling a hand chain operating a switch starts the light, and if the current fails and it goes out through any accident, an automatic device can be had for relighting it. Lamps are made for both direct and alternating current. A tube gives normally 3,750 hours' service, or the equivalent of 10 hours a night for one year. These Cooper Hewitt mercury vapor lamps are also manufac tured with tubes of a great variety of lengths and doublings, for use in factories, mills, ma chine shops, stores, photograph and moving-pic ture studios, etc. They bring illumination wherever it is wanted commercially, closely re producing daylight conditions and obviating strain on the eyes from glaring lamps. For outdoor use the Cooper-Hewitt quartz lamp is supplied, which employs a short tube of fused silica (or quartz glass), permitting a high tem perature and stronger current for brighter il lumination, as in railway yards, parks, etc. Such lamps have lasted for months without at tention.