This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
After having exposed the film in the camera, the next steps are to develop and make a positive film from the negative. The developing and exposing of the film for the positive are the same as in ordinary photography for making negatives and lantern slides, the only difference being in the apparatus for handling the long films. One of the simplest ways of developing a long film is to use a large tray in connection with a cross arm having upright pins around which the film is wrapped in a continuous spiral. A film 100 ft. long would require a tray 18 by 22 in., with pins set in the cross arm about % in. apart. This method of developing is shown in Fig. 4.
Ill: Fig. 4 - Cross Arms with Pins
A long film can be developed in a small tray by using two flanged wheels or spools mounted on a frame (Fig. 5) that holds them directly above the liquid in the tray. The spools have a wood core or center with metal sides of sufficient diameter to take in the length of film to be developed. One end of the undeveloped film is attached to one spool and then wound upon it, then the other end is passed through the guides, gelatine side down, and fastened to the other spool. The film is first run slowly through a water bath until it is thoroughly saturated, then it is passed through the developing solution again and again until the proper density is secured. The trays can be easily removed and others substituted for fixing, washing, hardening and soaking, the film being passed through each solution in the same manner. Before developing either negative or positive film, small test strips should be run through the solution so that the proper timing and treating of the full-length strip will correspond to the test strip.
A reel should be prepared for drying the film. This can be made of small slats placed around two disks to form a drum (Fig. 6 ) about l1/2- ft. in diameter and 2 1/2 ft. long. After the film has been passed through the various solutions and is ready for drying, it is wound spirally around on the slats with the gelatine side out, and the whole hung up to dry.
The printing to make the transparency is accomplished by a very simple arrangement. The negative and positive films must be drawn through a space admitting light while their gelatine surfaces are in close contact. A box may be constructed in several ways, but the one shown in Fig. 7 illustrates the necessary parts and their relative positions.
The sprocket A is placed directly back of the opening B which may be regulated to admit the proper light. The sprocket can be purchased from a moving-picture stock house cheaply, but if the builder so desires, one can be made from wood turned up about 1 in. in diameter, or so that the circumference will receive sprockets at points 3/16in. apart. The sprockets are made of metal pins driven into the wood. Two rows of them are placed around the wood cylinder about 1 1/8 in. apart.
Ill: Fig. 5 - Developing Long Films
The cylinder is provided with a small metal shaft at each end which turns in round holes or bearings in the sides of the box. One of the shafts should project through the side of the box and have a grooved wheel, C, attached. The sprocket cylinder is driven by a smaller grooved wheel or pulley, D, to which a crank is attached for turning. The relative sizes of these wheels are determined by the speed of the exposure and the kind of light used. A 3-in. or 4-in. wheel on the cylinder sprocket shaft, driven by a pulley about 1 in. in diameter, will be suitable under ordinary circumstances. The opening B may be adjusted by two metal slides which fit tightly in metal grooves fastened to the wood front. The metal grooves and slides can be made of tin and painted a dead black. The films after passing over the sprocket, fall into the bottom of the box, or, if very long films are to be made, the instrument can be used in the dark room and the light admitted only to the opening B, then the ends can be dropped into a basket or other receptacle at the bottom and the unprinted portions carried on reels above the box.
Ill: Fig. 6 - Drying Reel
The speed of the exposure and the width of the opening B can be determined by making test strips. This can be done by setting the opening B to a certain width and turning the crank for
10 or 15 seconds and counting the number of revolutions. The proper exposure can be easily attained by this method.
Ill: Fig. 7 - Printing Machine