International projectionist (Jan 1959-Dec 1960)

Volume 34 ^■■■MHHi INTERNATIONAL PROJECTIONIST JANUARY 1959 Number 1 New Insights Into the Carbon Arc By ROBERT A. MITCHELL We who have been interested in and actually operated the carbon arc for more than 30 years are here given a new insight into that which actually happens to produce a light source which, of all other illuminants available today, rivals the power of the sun. THE CARBON ARC is a "gaseous discharge" between combustible electrodes which are exposed to the air and to various magnetic influences. The unique character of the arc makes it extremely sensitive to the current and pressure factors of the electric power supplied to it. Fortunately for the projectionist, the projector carbons manufactured today are high-quality products capable of giving excellent results when burned under the recommended conditions. Not only is a good arclamp needed — one capable of feeding the carbons at a proper rate and maintaining constant positive-crater positioning— but also a rectifier or motorgenerator set specifically designed for the type and size of arc it is to energize. The importance of correct volt-ampere characteristics of the power supply cannot be overestimated. Unlike an incandescent light bulb or other appliance having a nearly constant electrical resistance, the resistance of a carbon arc decreases as the current (amperes) increases. This is, of course, the effect of heat, which increases the conductivity of the arc stream and the layer of gas which covers the positive crater like a thin film. Unless the power supply has current-limiting characteristics, a carbon arc becomes a virtual short-circuit. As the current increases, the resistance of the arc decreases to permit a further increase of current, and a condition of instability is quickly reached. The carbons become overloaded, and the arc sputters violently because the lip of the positive crater is burned away. If the power supply be a generator, too great a drain of current usually causes the machine to stop generating altogether. The windings may overheat and burn the insulation, necessitating costly repairs. Temperature of Arc Stream An unstable arc always gives a flickering light; and an overloaded high-intensity trim of carbons fails to produce the desired H-I effect. When the lip of the crater burns away, the luminescent substances emitted by the positive core are lost in the tail-flame. Low-intensity carbons spindle when overloaded. Arclamp carbons are essentially pure carbon rods containing special mate rials in a central core. Carbons intended for use in simplified H-I ( "Suprex" ) lamps are copper-plated for better current conduction. The arc is "struck" in every type of lamp by bringing the ends of the two carbons together and quickly separating them by a distance of % to % inch — the "arc gap." D. C. Arc Light Emission This procedure permits current to flow and produces a hot spark which volatilizes some of the carbon to provide the current-conducting arc stream of carbon gas and hot air. Most of the light emitted by a direct-current arc emanates from the crater, or cuplike depression, formed in the end of the positive carbon. The electrons which constitute the electric current always flow from the negative carbon to the positive through the hot conductive gases of the arc stream. The electrons are invisible. Fahrenheit temperatures in the various regions of the carbon arc output. INTERNATIONAL PROJECTIONIST JANUARY 1959