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MICROSWITCH
• — O
BAR
V PIVOT
(on camera)
Fig. 3. Schematic diagram of equipment arrangement.
The magnetic release mechanism is controlled electrically by a cam device (shown on the camera shaft in Fig. 1), which serves to time the release of the bar with the position of the film on the drum. This insures that the exposed strip of film will not include the overlap region where the two ends of the film are joined.
A delay circuit is adjusted so as to cause the shutter mechanism to operate during the time interval of from 5 to 20 msec during which the impact between bar and specimen occurs. The record on the film strip is calibrated by superposing a still photograph of the distance between the two knife edges when both bars are just in contact with the specimen.
The data on the film strip are read by direct measurements made with a microscope mounted on a movable base. The base is fitted with two orthogonal motions and can transverse a maximum of 4.5 in. in increments of 0.001 in. From these data the displacement-time relationship is plotted.
The camera body is made of J-in. thick aluminum alloy plates. Front and back views of it are shown in Figs. 1 and 2. All joints have been sealed
and the entire assembly painted dull black. The back of the camera body is mounted on hinges which permit easy access to the drum. The back is locked in position by four trunk-type fasteners and is light sealed by a i^-in. thick neoprene gasket cemented to its inside surface.
The drum was turned from a 2-in. cold-rolled steel plate and its finished dimensions are indicated in Fig. 4. It is mounted in the camera on a keyed shaft with a large hex nut to facilitate easy removal in the dark. The film is placed on the inner surface of the drum and held in position by two springmetal strips which grip the film strip at its edges. The drum accommodates a 48-in. length of film, which permits a slight overlap, and there is a useful length of approximately 40 in. of film.
In the drum periphery there are two f-in. radial holes that are used to focus the slit image on the film strip. There is a corresponding hole fitted with a sealing flap in one end of the camera body.
The drumis driven by a small induction motor rated at ^ hp at 900 rpm. Through a belt and pulley arrangement, the speed of the camera drum is reduced
46
July 1952 Journal of the SMPTE Vol. 59