c.ikHiiK Ti:i.i.i'ii()\y (K\ iiiaii i-oLucr. i.ixi.s \m 



To providi- Inr (>|H'r,iti(iii dI tlii' s\>trm wlu-n ilu- .iiU'iiu.iti<iii on the 

 powiT liiu- has hoi-ii inati-rialK' iiicrfasi-tl 1)\- liiii' fault conditions a 

 power ann>lifier is pro\i(lf(l. This aniplilii-r employs a 50 watt tula- 

 1211-A) and is placed in the eircuit i)y a simple swiichinjj o()eration. 

 When this amplifier is operated, the output of the iransitiitlinji circuit 

 is impressed upon the j;rid of the 50 watt tuhe and ampiilu'd to aj)- 

 proximately fift>- times its normal power output. 



In the present ty[X' of carrier system duplex or two way operation 

 is secured by the use of two dilTerent carrier frecpiencies, one for 

 transmission in each direction. .\s will he |)i)inted out later in llu- 



Fig. 8^ Rear View of Receiver Panel 



section on signalint; the lower fre(|uenc\- is always assigned to the 

 calling station. The transmitting circuit must therefore operate at 

 two different frequencies. This change is accomplished by the auto- 

 matic operation of the relay shown in Fig. (i. The operation of this 

 relay changes the capacity in the oscillating circuit, thereby changing 

 its frequency. The values of the two frequencies at which the trans- 

 mitting circuit operates are determined by the variable condensers 

 Fl and F2, Fig. 6, and certain fixed condensers which are connected 

 in parallel with the variable condensers. 



The receiving unit shown in Fig. 8 is extremeh- simple. It is not 

 tuned and the only control is the filament rheostat. It consists of 

 three 101-D vacuum tul)es operating respectively as a carrier fre(iuency 

 amplifier, a negative grid potential detector and an audio freciuency 

 amplifier. 



Two way operation is secured In- operating tiie transmitting and 

 receiving circuits at dilTerent fretiuencies and separating them by 

 means of filters. In the single channel systems this separation is 

 secured by a high pass filter and a low pass filter although in the mul- 



