ARC LAMPS The characteristics of arcs that are of special value for illumina tion are efficiency, ruggedness, adjustability, concentration of light source and the control of the colour of light. The necessity of trimming, the presence of undesirable products of combustion and the complicated mechanism are objections to them. The arc is largely confined to large-wattage units for outdoor illumination, to projection uses (because of its superlative crater brightness), and to photo-chemical uses where high efficiency in producing blue and violet light is necessary. Table I. gives a few of the important facts in the history of arc lighting.
Arcs may be divided into two major classes: (a) crater arcs and (b) luminescent arcs. The crater is the hot surface at the electrode tip from which the arc stream originates.
In the crater arc it emits most of the light. The positive crater gives about 90% of the light, the negative crater and the arc stream the balance. The energy expended in the arc stream is largely wasted. With the luminescent arcs considerable light comes from the arc stream in addition to that from the craters. To the crater arcs belong the open carbon arc, the enclosed carbon arc and oxide arcs. To the luminescent arcs belong the flame carbon arcs, magnetite arcs, mercury-vapour arcs and the tungsten arc.
The amount and quality of light from an arc depends on (I) the chemical composition of the electrodes; (2) the chemical composition, motion and pressure of the atmosphere around the arc; (3) the kind and amount of current and the voltage across the arc; (4) the magnetic field in and around the arc; (5) the nature of the ballast in series with the arc.
As the chief light source of a pure carbon arc is the positive crater, this crater area is a variable of great importance. The crater area depends on the composition and size of the electrodes, the current, the arc length and the chemicals in the arc. It has been found that, generally, with solid carbons the crater area increases
times when the current is doubled. With flame arcs the light increases faster when the current is increased. For good stability of the arc, it is important to conserve the highest temperature of the positive electrode; usually done through a smaller or pointed cathode. The carbon arc is the most stable of all arcs that operate on alternating current, because of the high temperature of the sublimation of carbon and the low thermal conductivity, and also because the flow of arc vapours, unlike that of the metallic arcs, is from the hotter positive crater to the negative crater spot, which is thereby heated. The temperature of the negative spot and hence
arc stability are improved by large current and high frequency.
The chemical corn position of the electrodes influences the brightness of the anode crater and the spectral nature of the light of the arc stream. Only materials having the highest boiling points, such as carbon, zirconium oxide, tantalum and tungsten, are suitable when the light is to be produced by the brightness of the craters on the electrodes. With flaming arcs the flame materials are carried from the anode to the cathode. The inclined-trim carbons are usually made with flame material in the positive electrode only, because its high heat is sufficient to fill the arc with the light giving vapours. The magnetite arc has a comparatively cold anode, made of copper. The cathode is composed of magnetite (elec trically conducting oxide of iron), titanium oxide which is the best light giver in these arcs, chromium oxide which decreases and regulates vaporization, and alkali salts for improving the arc conductivity. While in the case of the flame arc the salts are carried into the arc by vaporization from the anode, in the magnetite arc the materials are carried into the arc by vaporization from the cathode. An upper limit is reached in the use of large amounts of flame material because of (I) the increased energy required to boil the greater amount of material; (2) increased cooling effect on the arc stream; and (3) the increased obstruction of light by the condensation of the flame vapours as a dust around the arc. The chemical composition of the atmosphere around the arc affects its light materially. With tungsten arcs operating in low-pressure vapours such as titanium chloride the electrodes con tribute no light-giving vapours, but the atmosphere feeds the arc with light-giving chemicals. With the carbon arc, enclosures greatly increase the light. With the flame arcs, especially those giving light by chemical reaction in the arc shell, the reverse occurs.