11. a) (2) Define principal focus; conjugate foci. 



A moving-picture machine is to be designed to project the picture on the 

 film upon a screen 60 ft. from the film. If the image on the screen is to be 119 

 times the linear dimensions of the picture on the film : 



b) (2) How far from the film must the projection lens be placed ? 



c) (4) What must be the focal length of the lens ? 



d) (2) How many times as intense will be the light passing through the film 



as that falling on the screen ? 



12. a) (4) Make a diagram to show the dispersion of a narrow beam of sunlight 



by a triangular glass prism. 



b) (4) What is the explanation of refraction ? of dispersion ? 



c) (2) Explain carefully why the same blue cloth may seem to be of a different 



color when viewed by gaslight and by sunlight. 



Group V 



13. a) (2) What type of cell is best adapted to the ringing of electric bells? 



Why? 



b) (2) What is meant by the term local action as applied to voltaic cells ? 



c) (2) How may local action be reduced ? 



d) (2) What is meant by polarization of cells ? 



e) (2) How may polarization be reduced? 



14. Two resistance coils of 10 and 30 ohms joined in parallel are connected in 



series with a key, an ammeter of negligible resistance, and a battery whose 

 electromotive force is 21 volts and whose internal resistance is 3 ohms. A 

 voltmeter of high resistance is connected in parallel with the battery. 



a) (2) Draw a diagram of the connections. 



b) (2) \\1iat will be the ammeter and voltmeter readings when the key is 



open? 



c) (6) What will the readings be when the key is closed ? 



15. (10) Describe the construction and operation of two of the following : electric 



bell; telegraph key and sounder; telephone receiver. IDustrate by 

 carefully drawn diagrams. 



^o 



