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THE MOTION PICTURE PROJECTOR 189
the whole unit. By careful manipulation of these six levers it was possible to maintain a reasonably constant screen brightness.
The Low Intensity arc, having a surface brightness of 170-candlepower per square millimetre, consists of a relatively large diameter positive carbon, through the centre of which is now added a soft carbon core which has been found of great help in forming the required crater necessary to control the position of the light source. The negative carbon is now approximately 4-mra smaller in diameter than the positive but, since such a reduction would have previously limited the current carrying capacity, it is now also supplied with a core but, in this case, one which is coated with a thin copper deposit whereby the original current flow may be maintained and yet the negative may be kept sufficiently small to avoid blocking the path of the light emitted from the crater in the positive carbon.
The modern High-Intensity arc has a surface brightness of between 500 and 1,100 candle-power per square millimetre and such a source depends for its efficiency upon the filling used in the core of the positive carbon. It was found that, by filling the core with a particular mineral salts, chiefly cerium, most of this intense light was obtained from the burning gases and not from the actual carbon crater. However, it was found that, in order to control the flame produced by this arc, it was necessary to continually rotate the positive rod and this was accomplished either by hand, or later, by an automatic feeding mechanism.
A more recent method used to control the position of the flame and the crater in the positive carbon is by introducing a magnetic field in the vicinity of the arc. This field serves to stabilise the operation and, naturally, eliminates a considerable quantity of mechanism from the lamp house. The actual function of this magnetic field is still not fully understood, but it is quite certainly based upon the fact that magnetic fluxes exist around the carbons and their flames and, by applying similar fields in opposing directions, it is not only possible to separate the two flames but, if the field strength is increased, to completely extinguish the arc.
Projection Lenses and Screen Picture Distortion Projection lenses must be corrected for all types of aberrations if a clear, sharp picture is to be obtained. Some of these aberrations are briefly mentioned here to remind us that, in common with the camera lenses, these objectives are also of a very high order. Unless a lens is corrected for spherical aberration those rays which strike the edges of the lens will be refracted to a greater degree