TELEGRAPHY — HILLIS 193 



mitting units of the different channels and turns them to the line 

 in proper sequence. At the distant terminal they are picked up by 

 a receiving distributor and turned to the corresponding printers. In 

 order for the transmitter on chamiel 1 to send to the distant printer 

 on channel 1, it is necessary to synchronize the sending and receiv- 

 ing distributors and drive them at closely regulated speeds by syn- 

 chronous motors. 



Alternating current for the motors is supplied by accurately regu- 

 lated tuning forks. After the receiving distributor is brought in 

 phase, an accurate phase-correction circuit is applied which will hold 

 the receiving distributor in step with the distant sending distributor. 

 It is not necessary to transmit a special phasing pulse as the phase-cor- 

 rection circuit operates from line intelligence signals. Experiments 

 have been made in driving the sending and receiving distributors from 

 a common frequency source and using no correction at the receiving 

 end. Long distances between terminals has prevented the adoption of 

 this method of maintaining synchronism because of the lack of a 

 common power source. 



The Multiplex is operated duplex, as 2, 3, or 4 channels, depending 

 upon traffic load and circuit conditions. A standard operating speed 

 of 66 words per minute has been adopted in order to facilitate the 

 patching of channels between Multiplex systems. 



A 4-channel system, duplexed, working at 66 words per minute, with 

 4 sendings at each terminal, will provide a total message capacity of 

 528 words per minute. This is a considerable increase from 10 words 

 per minute of the original recorder used on the Baltimore-Washing- 

 ton circuit. 



The Teleprinter has ahnost entirely replaced the Morse method as a 

 means of operating lightly loaded circuits. Most of you are familiar 

 with this machine which sends from a slightly modified typewriter 

 keyboard and receives on either tape or page copy. The sending 

 and receiving units may be operated independently or in series. 



The Teleprinter uses the 5-unit code of the Multiplex with the 

 addition of a phasing and stop pulse. The phasing pulse is necessary 

 to start the receiving distributor with the transmitting cam and the 

 stop pulse stops both the receiving and sending units so they will both 

 start in phase for the next signal. 



Supplementing the Multiplex and Teleprinter, we have the Vario- 

 plex, Telefax, and Photofax. The Varioplex uses the high capacity 

 of the Multiplex system and by means of a control rack, a number 

 of reperforator racks, as many as 36 Teleprinter subchannels may be 

 operated over a single wire. 



Telefax and Photofax are Western Union developments of the prin- 

 ciples of facsimile. Telefax utilizes the pick-up of a reflected light 

 beam through a photoelectric cell which translates changes of light 



