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HANDBOOK OF PHOTOGRAPHY 



The power rating given for R is the minimum safe value. Some resistors, especially 

 the vitreous enameled type, operate at high temperature at their rated power. In 

 such cases using a resistor of two or three times the power rating specified (but of the 

 same resistance) will fulfill the electrical-circuit requirements and permit the resistor 

 to operate with smaller temperature rises. 



The circuit shown in Fig. 16B is useful where two identical lamps are used. The 

 double-pole double-throw switch puts the lamps in series for operation at low light 

 intensities and places them in parallel when full light output is obtained. The current 

 taken from the line when the lamps are used at full output is the sum of their indi- 

 vidual currents at full output, i.e., if two No. 2 lamps are used, the total current 

 drawn from the line will be 8.7 amp. When the lamps are in series, the current taken 

 from the line is about two-thirds of the rated output for any one lamp, or about 3 amp. 

 for the two No. 2 lamps in series. The switch, wiring, and fuses must, therefore, be 

 able to carry at least twice the current of each lamp used, or at least 9 amp. in the case 

 used for purposes of illustration. 



A method^ of controlling the light output of lamps (ordinary Mazda as well as 

 photo flood lamps) over a wide range of light intensities when alternating current is used 

 is shown in Fig. 16c. An autotransformer^ is used to vary the voltage applied to the 

 lamp. The line is connected to the two fixed terminals of the autotransformer, while 

 the lamp is connected between one fixed terminal and the varying arm. In com- 

 mercially available autotransformers, the voltage across the lamp may be adjusted 

 smoothly from zero to its full value so that the light output also varies smoothly. In 

 certain models these autotransformers also act as voltage-step-up transformers and 

 supply voltages in excess of the line voltage. This is frequently a valuable feature 

 where ordinary Mazda lamps are used, since it enables greater than normal light out- 

 put to be obtained, but with a decrease in life. It is not recommended that photoflood 

 lamps be operated above their rated voltage as the increase in light output would be 

 obtained by a considerable decrease in life, which is already quite short. 



Several sizes of photoflood lamps are available commercially. Their electrical 

 characteristics are given in Table VI. 



The high temperature at which the filaments of these lamps operate causes the 

 glass bulb to become hot enough to scorch paper or cloth which may come in contact 



Table VI. — Electrical Characteristics of Photoflood Lamps Operating 

 AT Normal and Half-normal Voltage 



with it. Consequently, these bulbs should be used in metal rather than paper reflec- 

 tors. These bulbs consume considerably more power than most lamps used for home 



1 The autotransformer method cannot be used on direct-current circuits. Attempt to do so may 

 result in blown fuses or burned or damaged autotransformer. 



^ Suitable autotransformers are made by: General Radio Co., Cambridge, Mass.; American Trans- 

 former Co., Newark, N. J.; United Transformer Co., New York, N. Y. 



