Journal of the Society of Motion Picture Engineers (1930-1949)

film left in the camera, and has a dial which can be set for the required film length in the next run. Figure 2 is a cutaway drawing of the first self-contained interferometer gauge ever built. It was designed to measure the internal pressure of rocket motors in the range of 0 to 2000 psi. To keep the hot gases of the rocket motor from destroying the diaphragm, the pressure is conducted to the diaphragm by a short oil line. By choosing the proper viscosity of oil and the proper size of line, the diaphragm can be critically damped. The oil line then acts as a low-pass filter so that the frequency response of the system is the frequency response of the oil line itself. By keeping the line short and making sure there is no air in the system, the frequency response can be held above 10,000 cycle/ sec. In the lower left corner of Fig. 2 is a blower fan used to keep the light cool. This is necessary because approximately 100 w of power must be consumed to get a sufficiently bright source. Between the two film spools is a slotted disc which is driven by a synchronous motor and puts timing marks on the edge of the film by interrupting the main light beam. The slots in this drum are of varying depths giving 5, 10 and 20 millisecond marks with increased lengths of line in each case to assist in analyzing the film. In the center of the camera and next to the film frame is a holder for a small cylindrical lens. This lens is not shown in the optical diagram (Fig. 1) as it is not an essential component, but it does serve to reduce the size of the image of the slot on the film, thus increasing the frequency response that can be read for any given film speed. Mounted with its roller on the take-up spool is a microswitch which is operated by the increasing diameter of the take-up spool. This switch interrupts the current to the drive motor and applies energy to the brake on the supply spool. Figure 3 is a picture of a model that was intended primarily for measuring pressures in blast waves, although it has proven to be a versatile camera and has been put to many other uses. The plate marked "Mounting for Quartz Diaphragm" is mounted flush with the surface over which the shock wave travels. This may be either the inside of the shock tube or the surface of the ground, as the experiment requires. The main features of this design are its ease of construction and its ruggedness. It is, of course, designed to stand the jars that it will receive when measuring shock waves. As in all very high speed cameras, there is a problem in keeping the end of the film intact as the driving motor comes to a stop. If the film is allowed to run free, approximately 1 in. is snapped off on every revolution, and at approximately 10,000 rpm, an appreciable section of the film can be destroyed in a very short time. To prevent this loss of record, two precautions have been taken. One is the microswitch which cuts off the power when the reel gets full, and the second is the two spring leaves which can be seen on either side of the take-up spool. These leaves are mounted so that, as the spool gets full, the film bears against the springs and acts as a brake to bring the motor to a stop in a very short time. With these two precautionary measures in operation the film can be used to within a few feet of the end of the spool without fear of losing a record. Figure 4 shows the latest 16mm camera design. It is intended to measure pressures in free air or anywhere else that a small size is important. The camera model shown in Figs. 2 and 3 has selfcontained power supplies and needs only to be supplied with 110 v a-c and a starting signal. However, this latest camera requires an external power supply as well as a starting signal. As can be seen from the picture, the case is extremely rugged and will stand pressures of 100-lb shock without being 372 November 1952 Journal of the SMPTE Vol. 59