International projectionist (Oct 1931-Sept 1933)

The name Preston R. Bassett and the term high-intensity arc are synonomous: no experienced worker in the projection field can think of one without thinking of the other. The accompanying article, a contribution by Mr. Bassett to a recent symposium on the carbon arc sponsored by the Illuminating Engineering Society, represents one of the finest contributions on arc lighting that feos come to our attention. THE HIGH-INTENSITY ARC AS A PROJECTION SOURCE Preston R. Bassett THE electric arc and the projection of light have always been closely associated. The first searchlight (1871), and the first motionpicture projector (1895). both utilized the carbon arc as the source of light. From those early days up to now, the arc in some form has always remained one of the essential projection sources. The arc using pure carbon electrodes held the field for many years undisputed. The reason for this is very apparent. Light projection involves two elements — a light source and an optical system. An optical system has a definite focus, and therefore an efficient light source for projection purposes must have its light output concentrated in the vicinity of the focus. The unit of measurement of concentration is intrinsic brilliancy which may be expressed in candlepower per square millimeter of the light source. Comparative Light Sources Of the various types of light emission, incandescence, or the temperature radiation of solids, gives the greatest intrinsic brilliancies. It was a very fortunate circumstance for light projection that directed current passing between two carbon electrodes causes a portion of the surface or crater of the positive carbon electrode to become heated to a temperature of about 3,700 degrees Centigrade, which is the volatilizing point of the element carbon. It is fortunate also that carbon does not melt at this point but goes directly from a solid to a gaseous state. This makes a clean source. The crater of the carbon arc at this temperature emits light of a brilliancy of 170 candlepower per square millimeter. There was no other light source available that could even approach onequarter of this brilliancy until about fifteen years ago. At this time there was a great deal of activity in the development and improvement of light sources of all types. The incandescent lamp with its coiled tungsten filament and inert gas was being concentrated and increased in efficiency so that it could enter the projection field. It entered not on the basis of competing with the arc in brightness but on the basis of simplicity— the elimination of working parts and the necessity of changing electrodes. The incandescent lamp, however, was limited practically to an intrinsic brilliancy of about 35 candlepower per square millimeter. Another development which came into use at about the same time as the concentrated-filament projection lamps was the high-intensity arc. The high-intensity arc is more than an improved carbon arc. It is a new type of arc using as the light source a material which had hitherto never been used. The high-intensity arc also brought more refinement in mechanism and operation than the old carbon arc. It resulted, however, in providing a light source having an intrinsic brilliancy of from 500 to 1,000-candlepower per square millimeter. It was the first source of light which could be maintained in steady form that exceeded the carbon arc in brilliancy, which it did by more than 300 per cent, in one step. [13] The Hall & Connolly Sjiol, a modern highIntensity projection light source The following is a table summarizing the brightness of projection sources: Candlepotver per sq. mm. Lime light 3-4 Tungsten Filament 20-40 Carbon Arc 150-180 High-Intensity Arc 500-LOOO In order to understand the action of the high-intensity arc, it is essential first to study the effect of amperage on the ordinary carbon arc. There is almost no mention in literature of the effect of current on the type of arc discharge. This current effect is so great that an experienced person can tell by observation, and with considerable accuracy, the amperes passing through an arc between the wide limits of 10 to 300 amperes. Varying Amperage Effects Figure 1 shows a series of arcs between ordinary carbon electrodes. In each succeeding sketch of the series the current is doubled. The progressive change of phenomena is evident. The 20-ampere arc is the familiar quiet little arc having the usual small yellow flame tip above the violet arc flame. The peak of the flame tip occurs about half way between the positive and negative electrodes. Doubling the current shows the violet arc flame almost unchanged, but the tail flame has increased in length and has shifted toward the positive electrode. Doubling again to 80 amperes causes the tail flame to lean back over the positive electrode so that its tip is no longer over the arc. It also increases in length to 4 or 5 inches. The arc is now apparently ^n