THE PRESIDENT : The next part of our programme is a paper by Mr. L. B. Marks, the rating of arc carbons and arc lamps. on ARC CARBONS AND THE RATING OF ARC LAMPS. At the last convention of the National Electric Light Association, a special committee on the rating of arc lamps submitted a report, which was adopted and ordered printed in full in the Standard Rules of the association. The report reads as follows: Recognizing the difficulty, if not impossibility, of measuring with any degree of accuracy the illuminating power of the arc lamp, and the great necessity for a more precise definition and statement of the obligation of the producer of electricity for illuminating purposes to the consumer thereof, be it 66 Resolved, That in the opinion of this convention what is ordinarily known as a 2,000 candle-power arc lamp is one requiring on the average 450 watts for its maintenance, measurements being at the lamp terminals where no sensible resistance is included in series with the arc; in case such resistance is used it must be excluded in the measurements of the voltage." This resolution naturally suggests the inquiry as to how far the candle-power produced depends upon the watts consumed; in the present paper, attention will be confined to the part played by the arc carbon in this important relation. It is, of course, well known that the so-called 2,000 candle-power arc really gives a mean candle- Table showing the hissing and flaming points, respectively, for mean current, 9.5 amperes. *L. B. Marks: "Life and Efficiency of Arc Light Carbons." Trans. American The +W. M. Stine: "Influence of Arc Light Carbons on the Candle-Power." 1895. In order to determine these values, a pair of carbons was placed in a lamp and burned for twenty to thirty minutes. After the points had been well formed, the carbons were gently brought together until the arc hissed. A voltmeter connected to the terminals gave the potential difference. The lamp was then adjusted for a silent arc, and the mechanism so arranged that the rod was held rigidly. The carbons were then allowed to burn away until they flamed, the voltmeter being carefully watched meanwhile. Before the flaming point was reached, the instrument would invariably fluctuate, due to "jumping" of the arc. At the flaming point, there was a marked drop in the potential, as indicated by the voltmeter. Great care was taken to secure perfect alignment of the upper and lower carbons. The results given in the table are an average of three different tests of each pair of carbons. The carbons were all intended for a current of from eight to ten amperes, nine and one-half amperes being used in the tests. The total length of a pair of carbons. was ten inches in each case. It will be noted from the table that the range, that is, the difference in volts between the hissing point and the flaming point, varies greatly with different carbons; thus, while carbon No. 4 (see table) has a range of only 11.4 volts, carbon No. 3 has a range of 34 volts. Both of these carbons are sold by representative American manufacturers, the former being a squirted petroleum coke carbon, and the latter. a moulded coke carbon. The hissing point of the cored carbon averages over ten volts lower than the solid. The hissing point of American cored carbons (Nos. 15 and 16) does not vary much from that of foreign (Nos. 17, 18, 19), but the range of the latter is fully ten volts greater than that of the former, and exceeds that of any of the American products tested, cored or solid. In the case of some makes of cored carbons not recorded here, it was impossible to measure the hissing point with any degree of accuracy, because of the deposition on the negative of small portions of the core thrown out by the positive at low potential differences. Carbons Nos. II, 17, 18 and 19 in table are of foreign manufacture, all the others being home products. The amount and nature of foreign matter in a carbon have much to do with the commercial efficiency of the latter, and determine to a large extent the range of the carbon. It has been the desire of central station men to obtain, and the aim of the manufacturer to produce, an absolutely pure coke carbon, thinking that this would, in a large measure, solve the candle-power question so far as uniformity is concerned. The writer is of the opinion, however, that an absolutely pure petroleum coke carbon, if such if such could could be commercially manufactured for arc lamps, would not meet the requirements of central station practice to-day. Carbons 12 and 13 (see table) are taken from two batches made with special reference to purity. It will be noted that one of them hissed at 53.7 volts, and the other at 52. To be operated at maximum efficiency, these carbons would require more than 55 volts at normal current, and under no conditions in practice would they give satisfactory service at less than 55 volts average--fully five volts higher than that of good commercial carbons. In connection with the potential difference required for maximum candlepower in a given case, it is interesting to refer to a test reported by Professor H. S. Carhart,* in which a pair of commercial carbons designed for use on full arc circuits showed a maximum efficiency at about 55 volts. These carbons at 10 amperes and 45 volts gave 450 candle-power, whereas at 8.4 amperes and 55 volts they gave double the candle-power. Thus, it is of the utmost importance to the carbon manufacturer to test not only the range of his carbons, but also the voltage at which they give maximum efficiency. The importance of steadiness in arc lamps has been very well shown by the introduction of cored carbons. Here, again, we are confronted with the question of candle-power in relation to the consumption of energy. If with a 450-watt arc we obtain a certain candle-power with efficient solid carbon sticks, we must make an allowance of at least from fifteen to twenty per cent for reduction in candle-power for the same energy on substituting an efficient cored carbon stick. Yet, for in-door illumination the consumer is better satisfied with the cored carbon than with the solid. Reference has already been made to current density. It would be entirely out of the question to designate a given current density for commercial arc light carbons, yet, in attempting to formulate a relation between candle-power and energy expended in the lamp, the question of current density must necessarily be considered. Schreihage deduced the law that for homogeneous carbons of the same make, the candle-power varies inversely as the diameter. He states that this relation, as is to be expected, is not completely satisfied in the case of the mean horizontal candle *Professor H. S. Carhart : "Maximum Efficiency of Arc Lamps with Constant Watts.' Proceedings of the International Electrical Congress, Chicago, August, 1893. + M. Schreihage: Centralblatt für Electrotechnik, No. 22, 1888 |