International projectionist (July-Dec 1934)

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July 1934 INTERNATIONAL PROJECTIONIST TAIL FLAME BALL SHAPED PORTION OF ARC LOWER LUMINOSITY — HIGHLY LUMINOUS PORTION OF ARC Fig. 3. 8-mm. a-c. high-intensity carbons : 80 amperes, 25% volts ; good operating conditions (5.7 mm.) in diameter. The arc itself consists of a highly luminous portion close to each electrode, and a portion of lower luminosity almost the shape of a ball extending about as far below the electrodes as above them and ending at the top in two well-defined short tailflames. It is interesting to note that the shape of the highly luminous portion of the arc near the electrodes approximates the shape of the intrinsic brilliancy curve across the electrode face, which was presented in an earlier paper1 and is reproduced in Fig. 4. This highly luminous portion of the arc close to the electrode decreases in size as the current is decreased, and becomes very small at the lower current-densities, as illustrated in Fig. 2. This result gives, of course, a much lower intrinsic brilliancy curve and less light on the projection screen. If the arc length is decreased it will hold essentially the same arc shape; if the operating conditions are favorable, until it reaches approximately 0.23 inch (5.8 mm.), or 24 volts. Fig. 5(A) illustrates the arc just before that point, with a good burning condition; and Fig. 5(6) the arc just after that point, with a shorter arc length and poor burning conditions. When the arc length is 0.23 inch (5.8 mm.) the arc stream begins to be turbulent. The two tailflames and the highly luminous portion of the arc close to the electrodes lose their identity, and the whole arc assumes a boiling and seething appearance. There is rapid flicker; the arc voltage and current are erratic; and in addition, at such very short arc lengths there is a noticeable s 5? 400 Fig. 5. 8-mm. a-c. high-intensity carbons : 80 amperes, 23 to 24 volts ; (A) short arc length, good operating conditions; (B) short arc length, poor operating conditions shadowing effect from the electrodes bons at 80 amperes will exhibit good themselves. burning characteristics for arc lengths If the length of the arc is increased between 0.23 inch (5.8 mm.) and 0.35 bevond that shown in Fig. 3, the form inch (8.9 mm.) , and from approximately of the arc will be sustained if the operat 24 to 29 volts. There will, however, be ing conditions are suitable, until the -,a noticeable change of light intensity be length is approximately 0.35 inch (8.9 tween those extreme limits, so that the mm I permissible range of variation in arc Fig. 6(A) shows the arc immediately length and voltage from moment to mombefore. and Fig. 6(B) after, such a ent is much less than the complete point has been reached. At and beyond range of satisfactory performance. This that length the arc has a tendency to be is discussed in greater detail subsequentswept upward, so that the lower part no ly longer bows down appreciably and the The limits of arc voltage, as ordinarily upper part and the tail-flame become measured at the incoming leads, will vary greatly extended. The highly luminous slightly depending upon the length of portion close to the electrode likewise spindle of the carbons, the lengths of the becomes distorted, as shown in Fig. carbons in the holders, and the resistance 6(B). The arc is unstable to such an extent that it will repeatedly jump back and forth between the positions shown in Figs. 6(A) and 6(B). Another arc condition that must be considered and if encountered, corrected, is shown in Fig. 7. The arc is of med of the holders themselves. At 75 amperes the arc lengths that will give good burning characteristics with the 8-mm. carbons are essentially the same as those for 80 amperes, and the arc voltage is approximately one volt lower. The 7-mm. high-intensity a-c. ium lenglh, and would ordinarily have carbons are rated from 60 to 65 amperes, the appearance of Fig. 3. but is disturbed The corresponding conditions for good by external forces so that it appears very much like Fig. 6(B), and has the tendency to snap back and forth between that position and the one shown in Fig. 3, causing variation of the current and voltage, flicker, and uneven light distribution. This condition may be caused by too strong a draft in the lamp, or by an unbalanced magnetic effect due to a poor arrangement of the current leads; or by other means that would tend to distort the arc. If we assume that the design of lamp house, the draft, and the arrangement of leads are such as to avoid the above conditions, the 8-mm. high-intensity a-c. car operation are an arc gap of 0.21 inch (5.3 mm.) to 0.31 inch (7.9 mm.), and an arc voltage of approximately 23 to 26 volts. The action of the high-intensity a-c. arc under various conditions has a direct bearing on the limitations of the mechanism for feeding the carbons. From the considerations discussed above it is apparent that such a mechanism must be able to feed the carbons at a rate up to 5.5 inches per hour; depending, of course, upon the current passing through the arc. It must also prevent the arc gap from varying more than 0.10 inch (2.5 mm.) or 0.12 inch (3.0 mm.) ; or, in terms of voltage, it must prevent the arc from varying over a range greater ._ ,^" ~^. / \ T Y T 1 OA0IU5 OF CQATEtt IN INCHES Fig. 4. Intrinsic brilliancy across crater a-c. high-intensity carbons ; 80 amperes Fig. 6. 8-mm. a-c. high intensity carbons: 80 amperes, 28 to 29 volts; face: 8-mm. (A) long arc length, good operating conditions; (B) long arc length, , 25V2 volts poor operating conditions