International projectionist (Jan 1959-Dec 1960)

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H-I arc is low and the current (amps.) high in comparison with the voltage and current of a normal L-I arc consuming the same amount of power (watts). Nearly all L-I carbon trims require about 55 volts for satisfactory operation. A 1650-watt L-I arc thus consumes a current of 30 amperes. A "Suprex" H-I arc, on the other hand, requires only about 37 volts for approximately the same amount of power consumption (1665 watts) in its less resistive anode layer, hence the need for a current of 45 amperes. (Watts = Volts X Amperes.) "Compression" of Gas Ball The success of the ordinary H-I arc as a projection illuminant depends upon the compression of the heat-excited rare-earth atoms in the positive crater, which is deeper than the relatively flat L-I crater on account of the greater current density. By confining the flaming materials to the crater instead of allowing them to escape at once in the tail-flame, a bright luminescent ball is formed which is responsible for about 75% of the total light of the H-I crater (the remaining 25% coming from white-hot carbon, as in a L-I arc). Compression of the flaming materials is accomplished by the influence of a carefully adjusted magnetic field. Such a field appears to act upon the luminescent gases like a continuous draft of air, pushing them into the cuplike crater. The desired magnetic field is supplied by the current flowing through the carbons in lamps employing angular trims, or by a magnet placed behind the lamp mirror when the carbons lie on the same line (coaxial trim). Small auxiliary magnets are sometimes placed beneath the arc in both types of lamp to assist the main magnetic field for more efficient burning. The action of the magnetic fields can be seen in the tail-flame, which assumes a definite direction when the arc is burning as the lamp manufacturer intended. Directed drafts of air may also be used to shape and control the flame of luminescent rare-earth atoms in the H-I arc gases. The Gretner Ventarc, commercially available as the Strong Jetarc and the National Ventarc, attains extremely high light levels by constricting the luminescent gases via air jets concentrically arranged around •Plain core LOW-INTENSITY -.■ ""mmm> rz± ■ Effect core Copper coating HIGH-INTENSITY The plain core of a L-I positive carbon consists mainly of compressed powdered carbon to center the luminous crater and prevent arc "wandering." Small amounts of potassium salts are also present to assist in the formation of a conductive arc stream. The "effect" core of a high-intensity positive contains, in addition to soft carbon, rare-earth compounds which produce a brilliant white flame or luminescent ball in the crater. Carbons for use in simplified lamps are copperplated for less resistance to the electric current. the positive burner and directed toward the end of the positive carbon. Compression of the arc results in increased brightness, and the cylindrical form of the light source permits the use of an auxiliary spherical mirror facing the large elliptical mirror for increased screen light. Ballast Requirements As stated earlier, the carbon arc is very sensitive to the volt-ampere characteristics of the current supplied to it, and will quickly "run wild" unless the available amount of current is limited in some way. This is because increased current decreases the resistance of a carbon arc and permits still more current to flow. The power supply must "dole out" the amperes very gingerly, cutting down on the supply every time the arc tends to "sneak" a few extra amperes. An overloaded arc is an unstable, flickering arc which the automatic carbon-feeding mechanism cannot regulate satisfactorily. The conventional multiple-arc generator is designed to supply more and more current as the electrical load increases. This is why such a generator can supply two arcs burning at the same time during changeovers. When the second arc is struck, the first arc, already burning and sending light to the screen, is not disturbed in the least — it continues to be supplied with adequate current at normal voltage. But this same valuable characteristic of the multiple-arc generator makes it impossible to obtain satisfactory arc operation by connecting the arc directly to the generator terminals ! The arc would run wild, the generator would become overloaded, and the resulting large counter -electromotive force in the armature windings would demagnetize the generator and stop the production or current. A current-limiting device — the ballast rheostat — is needed when a multiple-arc generator is used. There must be one ballast resistance for each arc, and each ballast is connected in series with the arc it regulates. And currentregulation by a ballast rheostat is automatic! As more current flows through the arc, more must necessarily flow through the ballast, inasmuch as the number of amperes flowing is the same in all parts of a simple series circuit. The greater the current flowing through the ballast, the greater its voltage drop, for its ohmic resistance, unlike that of an arc, is practically constant. The result is a decrease in arc current and a return to normal burning. The ballast rheostat functions continuously, of course, so instead of periodic "up's and down's" in arc current, normal steady current flows through the circuit at all times. It is necessary, however, to employ an amount of ballast resistance (ohms) greater than a certain minimum value in each case to avoid unstable arc burning. Low-intensity arcs having a voltagedrop (arc-drop) of 55 or 60 volts require 80-volt generators and the absorption of 20 — 25 volts by the ballast. A ballast resistance of from 0.6 to 0.9 ohm is normal. Simplified H-I ("Suprex") arcs rated at 35 — 45 volts require 50 or 60-volt generators and the absorption of at least 15 volts by the ballast. This amounts to a ballast resistance of 0.2 — 0.4 ohm. H-I arcs of higher power have the same requirements as L-I arcs, that is, a 20 or 25-volt ballast drop and correspondingly higher generator output voltages. Remember the rule: Gen. volts = Arc volts -) Ballast drop. Rectifiers More Economical The resistance of the DC power lines from the generator to the projector lamps should be considered as part of the ballast resistance: however, the voltage-drop occasioned by the transmission lines is ordinarily so very small that it may be disregarded. As an example, a 100-foot length of B & S no 0 copper wire has a total resistance {Continued on Page 21) INTERNATIONAL PROJECTIONIST • JANUARY 1959