Arc light

“ No light surpasses the arc light in brilliancy, excepting the magnesium light. There are few cities in this country and Europe that do not employ the arc lamp as a means of street, station, and large-area lighting, owing to its superiority as an illuminant and the wonderful policing effect it has upon the slum sections. ”

“ But since they have seen an electric battery and an arc light, they should be ashamed to ever declare "the sun is hotter than any terrestrial furnace," or that it is hot at all. The arc and incandescent lights are object lessons they ought to study, for they will see almost an exact representation of how all light, heat and vital force are created on suns and planets. ”

“ It may be assumed without danger that the softest light is that of the candle, but we are not living in the candle age, and have to deal with either gas or electricity as the main illuminating agents.
139. We have to consider the mercury arc light, the yellow flame carbon, the white magnetite and titanium arc—all of high efficiency, giving orange yellow in the flame-carbon to yellow and yellow white in the acetyline of the tungsten filaments. ”

“ Place a large lens—as large as the aperture—in the way, and focus the beam on yourself, and you will feel a heat equal to that of the arc light—about seven thousand degrees Fahrenheit—less what has radiated into the surrounding air.
"Now, get out of that focus, and place an opaque disk on the center of the searchlight aperture, and the beam will go forth with a dark center, like a hollow pipe of light and heat. And this dark center will have the temperature of the outer air, plus only what has radiated into it from the surrounding pipe. ”

“ Sometimes one, and sometimes a quick succession of luminous currents run from one end of the arc or bow to the other, so that the rays rapidly increase in brightness; but it is impossible to say whether the coruscations themselves are actually affected by a horizontal motion of translation, or whether the more vivid light is conveyed from ray to ray. The rays occasionally dart far past the zenith, vanish, suddenly reappear, and, being joined by others from the arc, form a magnificent corona or immense dome of light. ”

“ The carbon points of the electric arc-light are eaten away with tremendous rapidity in the very act of giving forth their illumination, and they need to be continually approximated and to be frequently renewed. ”

“ The two grand essentials now being at command, invention of electric light appliances went on rapidly upon two lines, eventuating in two systems, which became known as arc lighting and incandescent lighting. By 1879–80, the arc light was sufficiently advanced to meet with favor as an illuminant for streets, railway stations, markets, and any large spaces, in which places it became a substitute for gas and other lights. The essential features of the arc light are: (1.) The dynamo machine, situated in some central place, for the generation of electricity. ”

“ The rays from the electric arc, as it is called, either pass directly through the glass lens, or are caught by the parabolic reflector and shot back through it in an almost parallel pencil of wonderful intensity, which illumines the darkness like a ray of sunshine slanting through a crack in the shutter of a room. The search-light draws its current from special dynamos, which absorb many horse-power in the case of the powerful apparatus used on warships. ”

“ When [140] it is desired to keep the arc at the focus of a reflector, both carbons must be fed.
Fig. 213 shows a theatre arc lamp, used to throw a strong beam of light from the balcony to the stage.
Fig. 214 shows the arc lamp used as a search-light. The reflector throws a powerful beam of light that can be seen for miles; in fact, the light is used for signalling at night. Fig. 215 shows how search-lights are used at night on war-vessels.
[141]
CHAPTER XXIII.
X-RAYS, AND HOW THE BONES OF THE HUMAN BODY ARE PHOTOGRAPHED.
Fig. ”

“ Arc Lamps produce a light when a current passes from one carbon rod to the other across an air-space. As the current starts through the lamp, the ends of the carbons touch, and the imperfect contact causes resistance enough to heat the ends red-hot. ”

“ If carbons impregnated with various salts are used to colour or increase the light, the arc is called a chemical or flame arc. The carbons or electrodes may be arranged in line one above the other, or they may be inclined so as to project the light downwards or more in one direction. ”

“ Owing to the inequality in the distribution of light from an electric arc, it is impossible to define the illuminating power by a single number in any other way than by stating the mean spherical candle-power. All such commonly used expressions as “an arc lamp of 2000 candle-power” are, therefore, perfectly meaningless.
The photometry of arc lamps presents particular difficulties, owing to the great difference in quality between the light radiated by the arc and that given by any of the ordinarily used light standards. (For standards of light and photometers, see Photometer.) ”

“ Turning next to the consideration of the electric arc as a source of light, we have already noticed that the illuminating power in different directions is not the same. If we imagine an electric arc, formed between a pair of The arc as an illuminant. ”

“ The ultra-violet waves, which are obstructed by glass but transmitted by quartz, are the most effective. The source of light is an arc lamp. The light is passed through a lens of rock crystal to the receiver. ”

“ The arc lights were of 1500 to 2000 candle-power, and though every care was used to obtain the best carbons in the market and to keep the machinery going with the utmost regularity, these lights were always unsatisfactory owing to the changes in the colour of the arc and the illuminating power, the glare and flicker and the black and impenetrable shadows thrown by their own framework or by any intervening object. ”

“ This luminous arc is called the voltaic arc, and it excels all other artificial lights in brilliancy, a fact due to the extremely high temperature to which the carbon particles are heated, the temperature being, perhaps, the highest we can attain. It must not be supposed that in this brilliant light we see electricity: the light is due to the same cause as the light of a candle or gas flame, namely, incandescent particles of solid carbon. ”

“ In measuring the candle-power of arc lamps it is necessary to have some arrangement by which the brightness of the rays proceeding from the arc in different directions can be measured. For this purpose the lamp may be suspended from a support, and a radial arm arranged to carry three mirrors, so that in whatever position the arm may be placed, it gathers light proceeding at one particular angle above or below the horizon from the arc, and this light is reflected out finally in a constant horizontal direction. ”

“ A crater is formed at the positive tip and this is always characteristic of the positive carbon of the ordinary arc, although it becomes more shallow as the arc-length is increased. The negative tip has a bright spot to which one end of the arc is attached. By wasting away, the length of the arc increases and likewise its resistance, until finally insufficient current will pass to maintain the arc. It then goes out and to start it the carbons must be brought together and separated. The mechanisms of modern arc-lamps perform these functions automatically by the ingenious use of electromagnets. ”

“ There will be a continued demand for arc-lamps, for scientific developments are opening new fields for them. Their value in photo-engraving, in the moving-picture production studios, in moving-picture projection, and in certain aspects of stage-lighting is firmly established, and it appears that they will find application in certain chemical industries because the arc is a powerful source of radiant energy which is very active in its effects upon chemical reactions. ”

“ This is simply an arc lamp with reflectors behind it and lenses in front of it. The whole apparatus is pivoted so as to be easily made to shine in any direction. The function of the lenses and the reflectors is to collect stray rays of light and send them all out in the same direction. This is shown in Fig. 103 where for simplicity the lens is represented as a single piece. L represents a point of light which will naturally send its rays out in all directions as the radii of a sphere; m, m, m represents a bright reflecting surface which is given that peculiar curve called a parabola. ”

“ The mere inspection of the two angles enclosed between the index and the two beams suffices to show their equality; while if the graduated arc be consulted, the arc from 5 to m is found accurately equal to the arc from 5 to n. The complete expression of the law of reflection is, not only that the angles of incidence and reflection are equal, but that the incident and reflected rays always lie in a plane perpendicular to the reflecting surface. ”

“ The cost for labour and carbons is considerably reduced by the employment of the enclosed arc lamp, but owing to the absorption of light produced by the inner enclosing globe, and the necessity for generally employing a second outer globe, there is a lower resultant candle-power per watt expended in the arc. ”

“ Each opening of this moveable plate allows the light to pass during the same space of time, and the effect upon the sensitive surface indicates exactly the intensity of the chemical rays. The rapidity of the fall may be augmented or diminished by altering the inclination of the plane by means of a graduated arc, C, D, (Fig. 30,) furnished with a screw, E, by which it may be fixed at any angle. The same result may be obtained by modifying the height of the fall or the weight of the moveable plate. ”

“ An ordinary carbon incandescent lamp yields about six per cent, of light to ninety-four per cent, of heat, the arc lamp about ten per cent, of light, while the mercury arc has climbed to the enormous efficiency of almost half light to half heat. ”

“ The electric arc is suitable for lighting streets and for large buildings, but cannot be used for lighting houses. The light is too intense. One arc would furnish enough light for a number of houses if the light could be divided so that there might be just the right amount of light in [121] each room. ”

“ The inconvenience of orthogonal illumination, which certainly gives better results, is avoided in the coaxial apparatus. Care must here be taken, by using suitable dark-field screens, that no direct rays enter the observing system. The only sources of light are sunlight or the electric arc. The limit at which sub-microscopic particles are made visible is dependent upon the specific intensity of the source of light. ”

“ Thus then, when electrical discharges occur in the polar regions between the positive electricity of the atmosphere and the negative electricity of the earth, the magnetic poles of the earth would exercise a similar influence on the icy haze which is conceived essential to the evolution of the auroral light. Thus the arc seen by the observer would be that portion of the luminous ring above his horizon, varying with the distance from the pole. ”

“ In this case, an arc lamp or ordinary calcium light can be placed up in the loft, above the top of the cyclorama, and behind it. A little experimenting will reveal many striking light effects. If one light or lamp is not sufficient, others can be placed in various positions to reënforce [pg 89] it. As conditions vary so greatly, it is impossible to supply more concise directions. ”

“ Like a luminous ribbon passing insensibly from one shade to another with diminishing brilliancy, each band is composed of a considerable number of bright rays of different breadths, disposed with a wonderful symmetry, increasing with the power of the analyzing instrument and the luminous intensity of the electric arc; the broader bright rays doubling into finer rays, and new luminous rays appearing in the dark spaces that separate the bright rays. ”

“ It can also be proved by direct optical experiment that two rays of light, circularly-polarized in opposite directions, and of the same intensity, become, when united, a plane-polarized ray, and that if by any means the phase of one of the circularly-polarized rays is accelerated, the plane of polarization of the resultant ray is turned round half the angle of acceleration of the phase. ”

“ To thus examine the constituents of a beam of light a lantern, with a lime-light or electric light, may be used for throwing a constant beam; we may then produce an image of the cylinders of lime or the carbon points in the lantern on a piece of paper or a screen, and our eyes will tell us that this is an instance of how the radiations from any incandescent substance are competent to give us light. We receive all the rays to which our eyes are tuned and we see a white image on the screen. ”

“ On the other hand, the surfaces of those particles which have a higher refractive power than the medium will appear roughened, because, on account of the unavoidable irregularities on the surface, many of the emergent rays of light must strike at the critical angle and so suffer total reflection, and consequently those portions of the surface will be less illuminated, producing the phenomenon of apparent roughness above noted. ”

“ To study the phenomena of diffraction it is necessary that our source of light should be a physical point, or a fine line; for when a luminous surface is employed, the waves issuing from different points of the surface obscure and neutralize each other. A point of light of high intensity is obtained by admitting the parallel rays of the sun through an aperture in a window-shutter, and concentrating the beam by a lens of short focus. The small solar image at the focus constitutes a suitable point of light. ”