The Moving Picture World (1908)

74 THE MOVING PICTURE WORLD to vary the size and shape of his pictures, with sharp scissors he can cut the other openings, and use the most suitable arrangement. In this way much pleasing variety which will add to the attractiveness of an exhibition can be easily introduced. In brief, this is the process of making a lantern slide by contact. In a future article reduction will be discussed and a practicable method described, also special instruc- tions as to how to illustrate a lecture. The Electric Light in the Optical Lantern. No. i.—By C. M. H., in The Kinematograph and Lantern Weekly. . Every great boom leaves its mark behind it. The greatest boom the lantern world has ever seen is that which is still reverberating throughout the land—the boom of living photographs, and one of the results is a marked popularity for the electric arc light as an illumin- ant for the optical lantern. All kine-projection apparatus use up a tremendous amount of light, and nothing but the electric arc has been found capable of supplying that light in sufficient intensity for any but the very smallest screens. The greatest obstacle to the use of electric light for lantern work lies in the fact that it is in only a com- paratively few places of lantern entertainment that suit- able electric current can be tapped, and there are so many almost insuperable difficulties in the way of its production specially for the purpose, that such a course is, generally speaking, impracticable, and its consideration may be left •out of the argument altogether. However, this difficulty is fast disappearing, for every week brings the announce- ment of new supply companies being formed to light fresh districts electrically, while older institutions are ever in- creasing their range of usefulness, annexing whole towns to their own particular area, and sending the subtle fluid through new arteries, to carry light to many an outlying ;suburb. Let us see what are the essentials to the production of the electric arc light. First of all there is the supply of electrical energy which may be—and almost always is— produced by a dynamo, wherein it is converted from the mechanical form. Or an electric battery may be its source, in which case its derivation is chemical. Of course, electrical energy cannot be "created" any more than any other, and it may be of interest to note that the amount of mechanical energy required to sustain an arc light for lantern purpose is equivalent to about two horse-power. The electric current "generated" bv a dynamo or other source, is led to the point where its electrical energy is converted to some other form such as light, heat, or mo- tion,—by means of conducting wires, or "leads" as they are technically called, and it is where these leads come to a termination that the lanternist's part of the work com- mences. The function of dynamos is to divide, as it were, the electricity into two parts, which may be token to be pres- ent everywhere, or rather to create a plenitude of electric- ity at one point at the expense of another. It may be regarded as a kind of pump, which pumps this something we call electricity out of one place, where it leaves an emptiness, into another, which therefore becomes fuller than normally. Thus a stress is set up between two points which are always seeking to equalize themselves, so that directly a suitable path is provided by which the electric- ity can flow from that point where it is greater than the normal to supply the deficiency where it is less, it will do so, and it will continue to flow in a steady stream or cur- rent so long as the dynamo keeps on pumping. But, as it flows, it will do work, and if the path is a simple wire the majority of the electrical energy will be consumed in heating that wire. Suppose that at one point the wire is broken, leaving a gap, say of one-eighth of an inch, which is filled, not bv air, which is a non-conductor, but by intensely hot metallic vapor. This is a partial conductive, and the electricity will make use of it as a means of bridging across the gap but it is such a poor agent that it will experience difficulty in passing through, and a very great deal of its energy will be expended in overcoming this difficulty—energy which will raise the temperature of the gas to heat at which it glows with brilliant light, and which will rapidly vaporize the ends of the metal wire. Thus the supply of gas will be kept up, and so it will go on until the metal ends have been consumed—either melted or vaporized, or both—to such an extent that the current can no longer jump the widened gap, and all action comes to an end. The stream of electricity which crosses the space between the two "eltcrodes"—as the wire ends would be called— takes the form of a bow or arch, and from this circum- stance the name of "arc" light is derived. In practice, metal electrodes are quite useless, for the light would be most fitful and irregular, owing to the melting of the metal, and it would, moreover, be deeply colored according to the particular metals used, thus copper yields an intensely green light. Rods made of hard carbon are employed, and this material yields a most brilliant white light, part of which only—and a small part—is due to the actual arc, by far the greater quantity coming from the incandescent points of the carbon electrodes. The supply of partially conductive vapor between the two electrodes—whieh it will be re- membered is a point of vital importance', fbr-by its pres- ence the arc is made possible—is kept up at the expense of the electrodes themselves, which are thus slowly con- sumed. This gradual consumption has to be compensated for by continually advancing the carbon rods as they are eaten away, and this is the function which an electric arc lamp or regulator has to perform. Arc light regulators for the lantern and other similar optical purposes may De divided into two great classes, "hand-feed" and "auto- matic." The latter was strangely enough the first introduced, owing probably to the fact that an automatic regulator is absolutely indispensable for general lighting purposes, so that the possibilities of a hand feed lamp were lost sight of altogether, and for a long time it held the field undisputed, through the field in those days being a very limited one. Lately, however, the pendulum has swung to the opposite extreme, and it shows every sign of stay- ing there. Mr. Davenport, lanternist to the Society of Arts, was, I believe, the first to point out in a practical manner that a hand-feed lamp would not only answer all the requirements of lantern work, but had besides very many great advantages over the automatic variety. Since then, when the demand for electric light in the lantern was just beginning to be felt, many things have conspired to further its popularity. There "have been one or two bad—though it must be admitted somewhat irrational— lime-light scares, resulting in the enforcement of utterly unnecessary and vexatious restrictions upon the use of this illuminant. Electric mains have spread like a huge spider-web, with a marvellous rapidity all over the coun- try, and lastly many inventors, men of practical experi- ence, have come forward with new designs and improve- ments and additions until the hand-feed lamps have reached a high degree of perfection. ,' {To be continued with ilhtstrations.)