International projectionist (Jan-Dec 1939)

of both will be made, showing what occurs during each fractional part of a second. Figure 4 is a diagram representing the opening and closing periods of a two-bladed shutter having 90-degree blades diametrically opposed, that is, the width of the blades is equal to that of the light openings, during two complete revolutions of the shutter, which is represented as rotating 24 times per second. Each shaded section represents that portion of the revolution during which the light is intercepted by a shutter blade; each unshaded section representing the open sections during which the light is allowed to pass to the screen. The shutter rotates 24 times per second, therefore two open and two closed sections represent l/24th of a second, and each open or closed section l/96th second, in elapsed time. In Fig. 4 it will be noted that the sine wave of the 60-cycle current has been superimposed on the shutter chart, the relation of each half-cycle or alternation of the arc supply current to each shutter opening and closing being clearly shown. In this case each shutter opening and each shutter closing occurs in l/96th of a second, whereas each halfcycle of current occurs in l/120th of a second. Therefore the time values will appear in the ratio 96 to 120, or 4 to 5. The relative time value of each is shown during each open and closed period of the shutter, each section being divided, hypothetically, into five equal parts. During the first closing, the 60cycle half-cycle extends over four of these parts, the figure 4 indicating the time value of the alternation relative to the time consumed by the shutter during one complete interruption. All alternations above the zero line A-A' represent those alternations acting upon the carbon facing the reflector (which will hereinafter be referred to as the "positive" alternation acting upon the positive carbon) . The full force of the positive alternation will therefore be active in the projection of light. Those alternations below the zero line (which will be designated the "negative" alternation acting upon the negative carbon) will act on that carbon facing away from the reflector. A considerable amount of light produced by this alternation will be inactive for projection, due to the position of the electrode relative to the reflector, and we can only estimate its value relative to that of the carbon facing the reflector. It has been assumed that these values indicated by the letter "B" are in the ratio of 1 to .5, therefore all full positive alternations will have a relative value of 4, while the negative alternations a value of 2. Extensive experiment indicates that these values are correct. Figure 4 shows that during the first shutter opening %ths of a negative alternation and % of a positive alternation are projected, through the shutter, to the screen. During the second shutter opening ^th of a negative alternation and a complete positive alternation are projected. During the third shutter opening %ths of a positive and one-half of a negative alternation are projected. During the fourth shutter opening x/4th positive and one complete negative alternation are projected. Above the graph, in parenthesis, are given the comparative numerical values of each portion of the positive and negative alternations occurring during each shutter opening and closing for the two complete revolutions. These are titled "Alternation Light Values" indicated by the letter "A". Below the graph are shown the sum of the light values of both alternations occurring during each l/96th second the shutter is open or closed and indicated by the letter "C". The shutter openings are, in the case of 60-cycle current, shown as passing light values in the successive ratios of FIGURE 5 CURRENT FREQ..50 SHUTTER INT. FRE&.48 -(24 .5) (11.5 2) (22 1.5) (10.5 4) (20 2.5) (9.5 6) (l8 3.5) (S.5 8) 80" SO zcf 0 .25 .50 .75 I. SHUTTER REVOLUTIONS 1.25 1.5 1.75 A — .ALTERNATION LIGHT VALUE D~ PULSATION INTENSITY I N PCT.MlN. B — TIME VALUE PULSATION FRE&UENCY 2PERSEC. C AVERAGE LI«HT VALUE 3.5, 4.5, 4, and 3; after which this cycle is repeated. Since a complete cycle of varying light values occurs during two revolutions of the shutter, it follows that during 24 revolutions, or in one second, 12 cycles will occur, producing a 12-cycle pulsation upon the screen. Not only is the frequency of the pulsation of importance but also the relative intensity of each pulsation must be known and is of importance if the subject of frequency co-ordination is to be thoroughly understood. In Fig. 4 (lower chart) the comparative values of each bght impulse projected during each consecutive shutter opening is shown graphically. The resultant levels are shown to be of unsymmetrical form, a rapid rise occuring from the value of 3.5 to 4.5 and a more gradual decrease to a value of 3 shown during the period of the third and fourth shutter openings. In this and following charts, the maximum increase over the minimum value of intensity will be computed and the result shown in percent of increase and titled "Pulsation Intensity." In the case under consideration the minimum intensity is 3 and the maximum 4.5, or a pulsation intensity of 50%. Fig. 4 also shows the light value level during each l/96th second of the two complete shutter revolutions. There seems to be no definite order to these, a distinguishing feature of 60-cycle current. All other frequencies shown in subsequent graphs have a definite order of values during the two shutter revolutions. Following the same line of reasoning as in the case of 60 cycles, it will be found that the ratio of shutter opening time to the elapsed time of the 50-cycle alternation is in the ratio of 96 to 100, or 24 to 25, therefore each l/96th second of shutter opening may be hypothetically divided into 25 parts, over 24 of which the 50-cycle alternation will extend as shown in the first section of Fig. 5. The four successive shutter openings show a gradually increasing amount of positive alternation while a gradually decreasing amount of negative alternation is being projected toward the screen. The successive average values of these are in the ratio of 13.5, 14.5, 15.5, and 16.5, indicating that a gradually increasing light intensity is being built up to a certain point, after which it will gradually decrease to a minimum, this cycle thereafter being repeated twice during each second of operation. In Fig. 5, should the phase relation between the shutter and current shift 90 degrees to the right, the light pulsation value would be reflected as shown in the lower chart. A maximum light OCTOBER 1939 n