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1950 FLASHLIGHTING EQUIPMENT 307
the lamps will always operate at their correct wattage. The lamps are started by a high-voltage impulse circuit contained in the control unit, the impulse being applied to the lamp through an insulated hightension cable. The circuit produces a steep-fronted impulse of approximately 15 kv and will reignite the discharge even if the lamp has not cooled down completely so that inconveniently long delays usually associated with high-pressure mercury vapor lamps are reduced. The arc is ignited by opening and closing a spring-loaded striking switch on the control unit. This switch discharges a condenser through the primary of the pulse transformer and produces a high-voltage high-frequency impulse across the secondary winding. A high-frequency choke in series with one of the lamps and mounted in the lamp housing prevents the impulse from being short circuited by the low-impedance path through the supply mains.
Immediately the lamps strike, the lamp voltage indicated on the voltmeter falls to approximately 20 v and as they warm up the lamp voltage rises. When the total lamp voltage reaches approximately 100 v the starting resistance which limits the starting current should be short circuited by throwing the starting switch over to the "Run" position. After this the lamps will rapidly reach their final operating condition. Interlocking of the starting and striking switches prevents accidental striking of the lamp if the starting switch is in the "Run" instead of the "Start" position. The run-up process takes approximately 10 min.
The lamps are operated normally at 1000 w with a series ballast resistance. To flash the lamps a section of the resistance is short circuited by a contactor, the duration of the flash being predetermined by a resistance-condenser timing circuit. Selector switches controlling the power and duration of the flash are interlocked to give flashes of 3 kw for 5 sec, 5 kw for 2 sec or 10 kw for 1 sec, as required. The duration of the flash and the power in it are limited by temperature considerations; at 5 and 10 kw, melting of the electrodes is the limiting feature while at 3 kw the limit is set by heating of the quartz bulb. After the lamps have been flashed, an interval of at least 30 sec must elapse before they are flashed again, in order to prevent damage due to overheating. This interval is provided automatically by a timing circuit which must be reset manually by a push button before the lamp can be flashed a second time. This circuit cannot be reset until the necessary 30-sec interval has elapsed.
The desirability of preheating the lamps before taking a color photograph has already been mentioned. This is done by pressing the "Preheat" button in the control unit. Part of the ballast resist