AUTOMATIC PRINTING EQUIPMENT 411 



The required degree of accuracy in timing the various switching 

 operations involved in reversing the direction of transmission was 

 secured by utiUzing the rotating shafts of the distributors at both 

 stations to control a timing mechanism which determined the lengths 

 of the transmission intervals in the two directions.^ This timing 

 mechanism is essentially an electrical revolution counter which can 

 be set to count any desired number of revolutions of the distributor 

 shaft and close within a fraction of a revolution of that number the 

 circuit which controls the operation of the various contacts which do 

 the actual switching. As the distributor shafts at the two ends of 

 the cable are maintained in exact synchronism in a manner previously 

 described, the timing mechanisms will therefore also operate in 

 synchronism, and if at the time of setting up the circuit they are 

 started in the proper phase relation the correct phase relationship will 

 be maintained as long as the operation of the circuit continues without 

 interruption. The timing mechanisms are driven from the distributor 

 shafts through the medium of an electrically operated clutch which 

 when disengaged permits the timing mechanisms at all stations to be 

 manually set in their proper positions and started together in this 

 relationship by means of a starting impulse sent over the line which 

 causes the clutches to engage. 



In order to provide for transmission intervals of various lengths in 

 the two directions, the timing mechanism includes a number of timing 

 elements each representing a different division of the line time, any 

 one of which can be quickly selected at will by the movement of an 

 indicating lever to control the length of the transmitting and receiving 

 periods. 



Upon the completion of the predetermined number of revolutions of 

 the distributor the timing mechanism operates a direction control 

 relay, see Fig. 7, the contacts of which are arranged to operate and 

 cut off the transmitters, discharge the cable, and connect the amplifier 

 and the printers in properly timed sequence. The actual time con- 

 sumed in making all of the circuit changes necessary to reverse the 

 direction of transmission, measured from the time of transmission of 

 the last signal combination to the time of printing the first character 

 on the printer at the same station, is of the order of five seconds but 

 will vary somewhat on different cables according to the length of the 

 cable and the magnitude and character of the interference and earth 

 currents encountered. 



During the interval in which the actual switching operations are 

 taking place no signals are being transmitted in either direction so 



*A. A. Clokey, U. S. Patent No. 1,601,941. 



