International Review of Educational Cinematography (Jan-Dec 1931)

Record Details:

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— 995 — The modern machines which work in full daylight are essentially the same as the indoor type. The plant is surrounded by a chamber normally open to the light but closed by shutters during the actual taking of the picture. Lighting is effected by means of 6-volt lamps, working on accumulators, and suitable condensers. The difficulties encountered are mainly those of a purely practical type, such as the warping and splitting of constructive material owing to the alternation of wet nights and sunny days, dew upon the lamps and condensers in the early morning, to say nothing of insects finding their way into the works of the clocks and the mercury baths. The motors are still of the 2-volt type, although the manipulation of six or eight square feet of shutters is rather a strain upon their capabilities. One very efficient and easily constructed type has a field magnet of two bobbins, the coils connected at both ends by flat laminated iron bars shaped slightly to the contour of the armature. This latter is an inch diameter and an inch long, of the usual tripolar type. The steel spindle is conically pointed at the ends and works in phosphor bronze bearings. A smear of vaseline prevents rust but no oil is used. Coupling is carried out with thin spring bans, including countershafting for varying speed, and other purposes to be mentioned later. The technique of taking a picture is as follows. The pin wheel on the clock drivng the spring contact into the mercury makes a circuit which we will call No. i and the motor starts up and (i) allows the shutters to close, which they do of their own weight (2), removes by a flax line passing through light traps any dust shades, root covers etc. which may be covering the plant (3), plunges two wire forks into mercury baths one of which lights the lamps whilst the other completes a second circuit with the motor and battery (which we will call Circuit No. 2) (4) Takes the picture during which (5) the spring contact is drawn from under the pin of the clock, thus breaking circuit No. 1 and leaving circuit No. 2 to complete the action (6) as the taking of the picture ceases the covers of the object are allowed to fall into their original postiion (7) the spring contact of the clock is allowed to return — it of course coming back just above the particular pin which depressed it (8) the wire forks are raised thus cutting out the lights and breaking Circuit No. 2 (9). The shutters are now nearly open and the impetus of the motor completes their elevation. Obviously the two most critical points of the operation are stopping and starting. Failure to stop means the useless expenditure of whatever film is in the camera followed by as much current as is in the accumulators. The alarm bell would not of course come into action as the apparatus, from its own point of view, would be working quite satisfactorily. Luckily this trouble has seldom occurred. Salvation depends upon the fact that everything is started in as perfect a condition as possible, and such changes as ingl. 2