Journal of the Society of Motion Picture and Television Engineers (1950-1954)

or through relays control the events. Each cam operating point may be adjusted independently of the other throughout a range of 1 sec. This timer is much more accurate than would normally be required for photographic purposes as it was designed to initiate events on any part of a 60-cycle wave to an accuracy of ±5 electrical degrees. This accuracy is required for certain types of circuit breaker tests for which the timer is also used. This sequence timer, in conjunction with an Industrial Timer Corp. Model J-410 Fastax Control Unit commonly called "The Goose," has proved quite adequate for a wide range of photographic instrumentation. Selection of Exposure Choice of lens aperture for the effective recording of high-intensity arc phenomena is contingent upon which part of the phenomena is of primary interest. If behavior of the arc, i.e. its conducting path, is to be studied, extremely small integrals of exposure must be used in order that the relatively small (in cross section) conducting path may be defined and not masked by the surrounding volumes of incandescent gases. The resistance of these surrounding gases is high compared to that of the central core, consequently they carry only a small portion of the total current. If, however, it is desired to record the behavior of these incandescent gases and such post-arc phenomena as the arc trail, relative exposure must be greater, since the luminosity and actinism of these superheated gases are much less than those of the actual conducting core. It has been the authors' experience that the conducting core of arcs passing currents of from 5000 to 25,000 amp can be recorded on Super XX film at 7000 frames/sec, using an aperture of f/22 plus neutral density filters having trans mission factors of from 4% to 25% (depending upon arc current magnitude). Satisfactory records of post-arc phenomena have been obtained using Super XX film, film speed of 4000 frames/iec and apertures of f/22 when the arc trails were the result of a 10,000-amp arc. Figure 3 (A) shows a single 16mm frame taken with exposure adjusted to record the incandescent gases create?! by the arc. Note that no details of the actual conducting path can be seen. Figure 3(B) is from a record of an arc of the same magnitude but approximately 1/50 the exposure. Note that in this case details of the intensely luminous core may be observed. Color film may be used to advantage in studies of arc trail behavior and deionization times. Change in color of the hot gases is indicative of temperature and color film may show such changes within a limited range. Its inherent narrow latitude, however, makes dmi< •<• of exposure critical if a good record is to be obtained in the region of principal interest. Discussion Fred Metlen (Boeing Airplane Co.): In spark-gap work with Kodachrome, I run more than one camera at various speeds. By doing this, the correct exposed portions of each make a complete picture of the action. The high speed takes only the initial flash; the slower speed cameras photograph the important part not shown from the flash. Harold Levinton (formerly of Bonnevtilt Fairer Administration, who read the paper) : We had only one camera available which did serve to give us the data required. I might suggest that dual exposure levels could be achieved with one camera by using an image splitter with a filter interposed so as to reduce the exposure of one of the images. This would eliminate the necessity for speed correlation between two cameras. Along this same line, dual images obtained by using some form of optical system to simultaneously record two different operations for correlation might be feasible. Harrington and Ramberg: Photography of Electrical Arc.