EXTENSION OF THE TELEPHONE SYSTEM TO SHIPS 



151 



for the tour speech or telegraph channels in the center; and on the 

 gallery above the transmitting units, the coupling coils, loading 

 inductances, and so on, between the sets and the antennas. The 

 motor generator sets capable of supplying as much as five kilowatts 

 at eighteen hundred volts to each of the transmitting sets are located 

 in an adjoining room and controlled from the operating room. 



Fig. 4 shows the interior of one of the transmitting units. In the 

 shielded box at the upper right hand corner is the master oscillator 

 which sets the frequency to be used for that particular channel. 



Fig. 4 



Next to the left are two more shielded compartments, each of which 

 contains an amplifier. The last stage of this amplifier employs a 

 fifty watt tube. In the larger unshielded compartment are located 

 above, the six radio frequency power amplifier tubes. The reason for 

 introducing two amplifiers between the master oscillator and the power 

 tubes is to prevent any reaction from the attenna circuit back to the 

 master oscillator. By taking this precaution the frequency of the 

 master oscillator never varies more than fifty cycles in eight hundred 

 thousand. The lower set of shielded compartments, at the right, 

 contains the audio frequency telephone amplifiers which supply 

 currents to the six modulator power tubes shown in the lower par 



