eye to open wider without the sensation of glare; thus increasing the apparent illumination. Distribution. 1148 c. p. The distribution of the light in a vertical plane has been investigated by Messrs. Freedman, Burroughs, and Rapaport, whose results are quoted herewith. They found that the distribution in an inclosed arc lamp is not the same as in an open arc lamp. See Fig. 270. The maximum in the former is at an angle of 25 degrees below the horizontal, instead of 40 degrees. The intensity, after decreasing, reaches another high value at 40 degrees, but not as great as at 25 degrees. The probable explanation of this peculiar form of curve is, that at 25 degrees the light comes obliquely from the crater, but is not cut off by the negative. Descending, the negative cuts off more light; but the rays emanate more perpendicularly from the surface of the crater until another maximum is reached at 40 degrees. The reflection from the bulb, and the position of the arc in it, would also alter this distribution. Fig. 270. Light Distribution, Direct Current Efficiency. Tables XI. and XII. (Freedman) show the effect of a clear and an opalescent inner globe, the same being shown graphically in Fig. 271. The same investigators measured the loss by opalescent outer globes, which they found varied from 35 to 50 per cent, and which occasionally is as great as 60 per cent. They conclude that with currents of 5 amperes and with two clear glass globes of the best quality the watts per candle are about .5, with opalescent inner and clear outer, the watts per candle are about .6, and with both inner and outer opalescent globes the watts per candle are about .95, being mean hemispherical candle-power in all cases. Holophane globes, whose construction is explained on page 334, gave the same loss of light as clear globes. Whether the run is continuous or intermittent will make a difference in the life, although only slight. Theoretically the life should be less for the intermittent test that when the lamp is kept burning without any stops, and this is found to be the case. The stoppage allows fresh air to get into the bulb each time, thus increasing the consumption. When the current is thrown off a lamp, it is noticed that the carbonic oxide gas ignites with the inrush of air, and by a series of minute explosions causes a chattering of the gas-cap. Sometimes it burns with a quiet blue flame that lasts for five or ten seconds. To find theoretically the amount of carbon consumed with intermittent use we can calculate the weight of oxygen the bulb contains when filled with fresh air, and from this determine the amount of carbon burned before the admission of any more fresh air. Taking 4 hours as an average run, a lamp burning 140 hours would have 35 stops, equivalent in consumption to 5 hours run, and on this basis would consume carbon as if it had burned continuously for 145 hours. An airtight outer globe will increase the life; but it has a tendency to raise the temperature of the interior sufficient to warp or interfere with the action of some of the parts, although it will not do so with proper construction. Alternating inclosed arcs have also reached a high state of perfection. In principle they are similar to the inclosed arcs for direct current. The essential difference is in the use of one or both cored carbons, with consequently lower voltage and greater current. With solid carbons the long arc has a tendency to be extinguished, and the vapor supply of a core is required to maintain the conducting medium between the electrodes. Owing to the inclosure, which gives a stability and freedom from interference of air currents, it is sufficient to use one cored carbon, and it is of course indifferent whether this be the upper or the lower. It is not advisable to use two cored carbons, for reasons explained in connection with continuous current arcs, namely, the efficiency is more or less sacrificed, and the deposit in the bulb is increased. The length of arc is greater than in the direct current lamp, being about of an inch. At the start the arc may be as long as to" before the air in the bulb is consumed, or the resistance up to its maximum value. When hot, the usual current is 6 amperes, with a voltage at the arc of 70 to 75 volts. With 70 volts and 6 amperes in a 104 volt circuit, the apparent watts at the lamp terminals are 625 and at the arc 420, the actual watts being 445 and 390 respectively. The watts consumed in the inductive resistance average 35 to 45. This resistance usually consists of a coil in series with the arc wound on a laminated iron core, and mounted in the trimming of the lamp. By connecting the terminals to different portions of this coil, the reactance may be greatly varied, so that the lamp is capable of a wide range of adjustment for various circuits. As a rule the reactive coil can be adjusted to maintain 75 volts at the arc for circuits varying in voltage from 100 to 125, and in frequency from 60 to 133 cycles per second. A striking advantage of the inclosed alternating arc is its freedom from the hum that characterizes open alternating arcs. This is due to two causes. In the first place, the mere inclosure in a fairly well-sealed bulb reduces the noise, but the action of the bulb in keeping the gases hot is the more potent factor. It will be recalled that in the case of the open alternating arc the hum was produced by the rapid expansions and contractions of the arc stream following the waves of current. When, however, the arc is surrounded by a heat-retaining envelope of glass, the gases at the arc do not contract as violently with its instantaneous extinguishment, hence the amplitude of the vibrations, and the consequent hum, are much reduced. Another source of noise in alternating lamps was the vibration of the laminated iron of which all magnetic parts are constructed. The thin sheets alternately repelling each other, and losing the repulsive force, are sent into violent vibration, which readily communicates itself to the whole lamp, with an effect like that of a sounding-board. Since by the modern method of inclosure the noise of the arc itself has been nearly eliminated, corresponding efforts have been made to reduce the hum of the iron. By clamping the core of the reactance coil and magnet cores at a great many points, the iron is held too firmly to vibrate. The iron parts are then supported entirely on springs and rubber, both in light compression, so that the vibrations are not communicated to the lamp frame. Tight inclosure of the whole lamp completes the deadening effect, so that modern alternating arcs are made nearly noiseless. The life of the alternating arc as usually constructed is much less than that of its continuous current congenitor. Owing to the complication in the mechanism caused by feeding both carbons simultaneously, and the difficulty of feeding through both ends of a bulb, the alternating inclosed lamp is usually constructed so as to feed only the upper one. But in an alternating lamp both carbons are equally consumed, and it becomes necessary either to make the lower carbon excessively long or to shorten the life. The latter is considered preferable, and the average life is about 80 hours with ordinary inclosure. For this an upper carbon of 91 × 1, and a lower one of 6 x inches are usually employed. CHAPTER XV. ARC LAMPS. Carbons. Manufacture. The performance of the arc light is so largely dependent on the quality of the carbons employed that some knowledge of their method of manufacture is of great assistance. Many of the discrepancies that have been found in laboratory experiments and commercial work are due to the fact that different kinds of carbons were employed. Carbons are of two kinds, according to their mode of manufacture, molded or forced. The molded carbon, as its name implies, is shaped in a steel mold. The forced carbon is squeezed while plastic through a circular orifice. The preliminary stages of treatment being similar, a single description will suffice for both. Various materials have been employed; but the most prominent is petroleum coke, which is a product obtained in the distillation of paraffin. Other materials, such as gas-coke, lamp-black, are also utilized for this purpose. The material is first crushed, then placed in retorts heated to a high temperature for 10 to 50 hours according to the result desired, thereby driving out moisture, and imparting the quality of conductivity. The carbon is next ground to a fine flour in mills, and then bolted. The carbon flour thus produced is put in mixing kettles or pans combined with the "binding material" consisting of pitch which has previously been crushed. These pans are kept warm, and the entire mixture is constantly stirred by hoes or other means for a period of fifteen minutes to an hour. The heat causes the particles of pitch to attach themselves to the particles of carbon. The mixture is then cooled, and again crushed, ground, and bolted, so that a flour of uniform grain is produced. Molded Carbons. From this point the treatment of the material depends upon whether molded or forced carbons are to be produced. If the former, the material is brought to men |