The Moving Picture World (1908)

156 THE MOVING PICTURE WORLD Company, whose cameras were-held not to infringe the Edison patents. Heretofore Edison has been the aggressor in the fight for supremacy of the camera patents, and Biograph have always successfully held their own, every right being con ceded them. Now, by the above action, it will be seen that the tables are turned, Biograph carrying the war into the enemy's camp. The Electric LigKt in the Optical Lantern. No. 4.—By C. M. H; Continued from page 114. In the construction of an automatic lamp, the arma- ture working between the two magnets is seldom con- nected directly With either carbon; it is more usual to let it actuate a brake, and to so arrange the carbons that they will come together by their own weight whenever the pressure of the brake is slackened, as it is when the arc gets too long, and the current in the main magnet gets proportionally weak. But enough has been said to show what delicate mechanism is required in a satis- factory automatic lamp, and it will not be difficult to understand that such mechanism may easily get deranged in unskilled hands, and that pretty extensive electrical knowledge would be necessary before it could be put into order again. Such a lamp, of course, if properly made, is an expensive article, but it is not at all necessary. A lantern must always have an operator in attendance upon it all the time it is at work, and a simple arrangement for holding the two carbons in such a manner that they can be conveniently fed together by hand at intervals whenever the distance between them is becoming too great, is all that the lanternist requires, for this hand regulation is only necessary once in every two minutes or thereabouts, and is no more trouble than the turning of a lime. Besides, it is a very great advantage for the lanternist to have the whole thing under his own control, instead of being dependent on a number of factors whose working he cannot see, and probably would not under- stand if he could, So much for the mechanism by which the carbons are held, and the distance between their extremities duly regulated. We can now pass on to a consideration of the best positions of the carbons them- selves so that they should yield the greatest possible amount of light, and send it in the direction in which we require it to go for our particular purpose. It must be remembered that the light emitted by the electric arc has, as it were, three separate sources of origin. First, and least brilliant of all, there is the actual arc itself—the band of light which marks the passage of the electric current across the space between the elec- trodes ; secondly, there is the light from the incandescent point of the negative carbon; and thirdly, and by far the most important, there is the light from the crater of the positive carbon. For all practical purposes the two former need not be taken into consideration at all, for the small quantity of violet-colored light which is due. to the arc itself, although of great actinic power, photo- graphically speaking, is in such insignificant proportion that it has little effect upon the total, while the incan- descent negative point is also of little account. So it will be seen that we have to deal with a source of light —the crater at the end of the positive carbon—which is barely, a quarter of an inch across, and it is not necessary to point out to lanternists.that this is- just about as near to the ideal of perfection, in lantern illuminants;. as any that could be "found. However** some "ineahs must be adopted for causing this little crater to take up its posi- tion on the side of the carbon rod so that its" light should be projected into" the lens system, and not all - round equally, as it is in an, ordinary street larhp. The manner in which this consummation is brought about is simplicity itself. The light-giving crater of the positive carbon, it must be remembered, forms at that point where the stream of electricity leaves it to jump across that space which separates it from the negative, and as elecricity always chooses the path of least resis- tance, this jumping^across occurs at those, points of the two carbons that happen to be nearest to one another. Now, if the negative carbon is shifted about half its diameter in front of the other, the arc will form between the front edge of the positive and the back of the nega- tive electrode. Consequently- the crater which always forms just opposite the nearest point of the latter, will take up its position towards the front of the. upper car- bon, and being tilted upwards to a certain extent, will throw nearly all its light in the required direction. As a further aid towards the same consummation, the whole apparatus is generally tilted backwards through a small angle, as shown in Fig. 5, where the. forward displace- ment of the lower carbon, with regard to,the other, and the position of the positive crater, to give the most effi- cient results, is also set forth. , Different workers have different ideas as to the amount of backward tilting which it is best to give to the lamp, and, of course, the greater the rake or tilt, the less it will be necessary to displace the positive carbon., behind the other, so that as a deficiency in either sense can, within reasonable limits, be remedied by a compensatory plenti- tude in the other, it is difficult to say which is the best balance of the two factors. However, when I experi- mented in this direction, I decided upon fifteen degrees of backward tilt as giving-the best results in my hands, and I have never had occasion- to alter it. It should be noticed that too great a displacement of "one carbon be- hind the other will seriously impair the steadiness of the light. It seems to me that the tilt of the lamp should such a displacement- of the carbons as to make the light be as little as possible, provided it does not necessitate burn unsteadily. I have already said that the electricity in an arc lamp flows between the two nearest points that the greatest amount of action occurs. It may be supposed that the consumption of the carbon is quickest in those places, and they will not long remain the nearest points. When they are burnt down to a certain extent, they naturally be*