Theatre Catalog (1949-50)

ny) fe) fe) oO BRIGHTNESS~ C/MM2 8 (e) ARC CURRENT—AMPERES FIG. 4 Comparison of a:rand water-cooled operation for new higher-current 13.6-mm “‘high-brightness’’ carbon. conductivity with freedom from excessive corrosion and rapid wear in service. Copper most economically fulfills these requirements in so far as the negative head is concerned. However, this same material fails because of excessive wear in the positive head. This is because copper is plated from the jaws onto the carbon, and this then scores the jaws as it is dragged around with carbon rotation. The reason this destructive effect is confined to the positive jaw, while the very similar usage in the negative gives no trouble, is believed to be associated with the direction of current flow, and the rectifying action of the copper-oxide and sulfide films which tend to form along the copper-carbon contacts. These films are conductive in the direction of current flow from carbon to copper in the negative holder, but they tend to block current flow from copper to carbon in the positive. Since the distance between the jaw and the carbon is so short, a contact drop of only 1 volt produces a gradient of perhaps several thousand volts per millimeter across the rectifying film. This is sufficient to rupture the film and draw copper ions across the gap to be neutralized on the carbon. Silver oxide and sulfide, on the other hand, are good conductors with no rectifying pronerties, and so silver is free from this dificulty. The jaws illustrated here have FIG. 5 Comparison of airand water-cooled operation for 13.6-mm super high-intensity, projector carbon, 300 . ARC CURRENTAMPERES operated several hundreds of hours, many of them at high currents from 300 to 500 amperes, with no significant wear and every indication of prolonged satisfactory performance. For purposes of securing comparative data, it is necessary that certain operating conditions be held constant. Factors determining the choice of these conditions in the tests to be described were as follows: The speed of rotation of the positive carbon was chosen at 15 revolutions per minute. However, the exact speed is not critical, so long as it is above the minimum required to insure a straight crater face. In the test lamp used, the angle between the positive and negative carbon is adjustable over a wide range. This angle is not ordinarily critical over a range between about 45 and 60 degrees of the negative-carbon axis below a horizontal positive. For the tests to be described, the halfway value of 58 degrees was chosen. At shallower angles, the positive tail flame is thrown objectionably close to the upper jaw, the arc is less stable, and it is more difficult to hold a straight crater. At steeper angles, the negative flame tends to pass in front of (rather than into) the crater, so that the are is more difficult to control, at least without the aid of an auxiliary magnetic field. In order to insure optimum cooling, the protrusion of the positive.carbon beyond the jaw should be held to as small a value as possible consistent with adequate jaw protection. A protrusion of 1% inch was used with 16-mm carbons and only % inch with 9-mm carbons. The use of a small negative carbon with a short protrusion contributes importantly to a stable are at high currents. The small carbon spindles to a sharply defined tip area, which is completely and stably filled with the negative flame at a current density of approximately 30 amperes per square millimeter. (This compares with a positive-crater current density of between 1 and 3 amperes per square millimeter.) It is obvious that there is much less freedom for are wandering here as compared with the comparatively blunt point formed on the much larger plated negatives conventionally employed in heavy-current service. The advantages of the small water-cooled negative are more pronounced as the current is increased. The determination of the maximum performance of a given carbon is dependent upon the choice of a maximum operating current. This was chosen at a value a little below that which resulted in unstable operation. Over a wide range of sizes and types of positive carbons, the same 5 -inch water-cooled negative was employed, giving very satisfactory oper ation at all currents from 90 to 500 amperes. ; Positive carbons from 9 to 16 mm 17 diameter have been specially designed to take advantage of the efficient cooling provided in the apparatus shown in Fig1. The first of these, a 13.6-mm carbon for operation. at 290 amperes, Was Cescribed in an earlier paper.’ One outstanding features of this type ° 949-50 f car THEATRE CATALOG 1!