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12. a) In making a photographic print by an electric Ught how will the illumina- 



tion at a distance of 5 ft. compare with that from the same light at a 

 distance of 15 ft. ? 



b) If an exposure of 10 seconds is correct at a distance of 5 ft. what should be 



the time of exposure at a distance of 15 ft. ? 



c) What is the physical difference between "red light" and "blue light" ? 



Group V 



13. Describe in detail a method for determining the resistance of a piece of wire, 



representing the scheme of connections by a diagram. 



14. In an apartment supplied with direct current at 110 volts there are two 



40-watt lamps and a heating unit connected in parallel. In one week the 

 lamps are burned 20 hours each and the heater is used 25 hours. The 

 total use of energy is 7,100 watt hours. 



a) How many watts are used by the heating unit ? 



b) How much current passes through the heating unit ? 



c) What is the resistance of the heating unit ? 



d) What current flows when both lamps and the heating unit are in use ? 



15. A direct-current dynamo furnishes a current of 50 amperes at 500 volts to a 



line having a resistance of 2 ohms. 



a) How much power is generated by the dynamo ? 



b) How much power is deUvered at the other end of the line ? 



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