International projectionist (Jan-Dec 1939)

changeovers are not employed. The single-coil type can be operated from a hand changeover bar or by the hand dowser located on the lamphouse cone. Arrangements have been made to adapt the magnetic changeover to any device which may be employed by the individual theatre. While the voltages of the commercial frequencies (220-380-440-550, etc.) are not suitable for direct connection to the arc, nor is that of the converter, the static transformer, besides being a distributing means, also changes these voltages to the proper value for use in the arcs. Figure 2 shows the converter proper, consisting of the rotary transformer (located below) driven by the small motor mounted together so as to form one compact unit. Power transmission is by a heavy vee-belt designed to transmit three times the power required. As shown in Fig. 3, the commercial frequency current enters the primary or stator of the rotary converter, causing a rotary magnetic field within which the secondary (rotor) of the transformer is revolved in a direction opposite to the rotation of the field. From the rotating secondary of the converter the arc current is transmitted through brushes and collector rings and sheathed wires to the control unit, where it is transformed to the proper voltage for direct use in the arc and switched to the proper arc at the will of the projectionist. The converter is constructed of only such units as will have very long life. There are no commutators to cause trouble, and the load on the bearings is so slight that possibility of trouble from this cause is negligible. The power system is so designed electrically that heavy overloads are automatically made impossible. Beyond a predetermined maximum current output the current decreases through reactance when attempts to further increase the current is made. This safety feature positively prevents overload of the converter beyond safe Hmits. • The Control Unit Figure 2 shows the complete control cabinet which is connected, through flexible metal conduit protected leads, to the junction box of the converter as shown in Fig. 3. The function of the unit is (1) to receive currents of single-phase, both commercial and co-ordinated frequencies, from the power line and converter, respectively, and transform them to the proper voltage values for use in the arcs; (2) provide adjustments for varying the voltages of both commercial and co-ordinated frequency currents supplied to the arc, and (3) provide switching means for transmitting the current of the proper frequency to the arc desired at the will of the projectionist. Attached to the control cabinet and connected to the transformer is the magnetic switch for current distribution to the two arcs. This switch has two separate coils which are connected to the respective changeover switches located at the projection machine. The control unit, which is very compact and occupies very little space (7 x 18 x 19 inches), can be placed in any convenient location. For instance, the converter which is also very small (18 x 21 x 12 inches), and is quiet in operation, can be located in the projection room with the control unit in front or to the rear of it, or the control cabinet can be placed on a shelf above the converter proper. If it be desired to locate the converter outside the projection room and the control cabinet in the loom, only three wires are necessary to interconnect the two units. The Cyclex system is exceedingly flexible, but due to the very low arc voltage it is preferable that short wires of ample size connect the control cabinet to the arcs and that all connections be very secure. : | i During the writer's exhaustive study of the cause of a.c. flicker on commercial frequencies, numerous charts and graphs were made showing the results at. many frequencies, commercial and otherwise. By means of these graphs, which will later be explained in detail, it is possible to analyze the resulting pulsation at any frequency. More important, however, is the discovery that by properly co-ordinating the component light impulse frequencies and, in turn, combining these with other factors of definite values, we can produce what we will call "Cyclo-Harmonic Light Projection." This term is applied to a condition existing when light impulses of certain frequencies are passed through a shutter having definite characteristics which intercepts the light beam at a rate relative to the lightimpulse frequency. When such a con dition exists there is a complete absence of secondary pulsations in the projected light. Light from such an arc is steady and particularly adaptable to the projection of motion pictures. The subject of co-ordinated light impulse frequencies can be explained in other ways, of course, but the simple graphic method seems the most appropriate. In motion picture projection there is one factor which is almost universally standard, that is, a shutter having two blades of 90 degrees each, revolving at a speed of 1440 r.p.m., or 24 revolutions per second. Any light intercepted by the shutter will, of course, have a screen frequency of 48 cycles per second, which is beyond the frequency of ordinary perceptibility. Only one-half of the light striking the shutter would be allowed to pass through, the other half being intercepted by the two blades. All other factors remaining the same, if 50 cycle current be supplied to the arc, not only will there be the primary light frequency of 48 cycles, but upon this frequency there will be superimposed a secondary pulsation, the frequency of which will be the differential between the arc supply frequency and the shutter interception frequency, or, in this specific case, a frequency of 2 pulsations per second. Likewise, if 60cycle current be supplied to the arc, the differential of 12 will result in a 12-cycle pulsation. While the 2-cycle pulsation of 50cycle.arc supply current appears on the screen as a relatively slow pulsation, the 12-cycle pulsation of 60-cycle current causes a violent flicker. Both frequencies, 50 and 60 cycles, are wholly unsuited for motion picture projection where good projection is expected. Since 50 and 60-cycle currents are both commercial frequencies used in the U. S. and other countries, an analysis FIGURE 4 CURRENT FREfl. 60 SHUTTER I N T. F R E a 48 (4'J5) (1.5 2) (2 1.5) (.5 ^) (2 l) (3 l) (I 3) (l 2) 80 60 D— 40 20 o 25 .50 .75 I 1.25 SHUTTER REVOLUTIONS 1.5 1.75 D PULSATION INT. 50 PULSATION FREQ. 12 10 INTERNATIONAL PROJECTIONIST