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of the open compartment. These two sets of six tubes each are con- 

 nected together to secure constant current modulation. The output 

 of these twelve tubes is led to terminals on the output of the trans- 

 mitter unit at the left. To secure cooling in hot weather, a fan is 

 installed below the power tube compartment. In the extreme left 

 compartment are shown choke coils in the power circuits, and at the 

 extreme left on the outside are circuit breakers and two handles for 

 operating the tuning and coupling apparatus in the gallery above. 



Fig. 5 shows one set of radio frequency apparatus in this gallery. 

 The two inductometers at the left are for coupling and tuning, and 



Fig. 5 



the large condenser at the right, the plates of which consist of brass 

 frames covered with copper window screen, is inserted in series with 

 the antenna. This capacity together with the inductance immedi- 

 ately above it stiffens the antenna circuit and increases the frequency 

 selectivity to prevent radio frequency interaction between the several 

 antennas. 



The telephone switchboard shown in Fig. 6 is a special type of 

 P. B. X. (private branch exchange), constructed to provide the 

 necessary shielding and to include telegraph oscillator, phantom 

 coils, and other special apparatus. This switchboard provides for 



