412 BELL SYSTEM TECHNICAL JOURNAL 



that neither of the distributors will receive any correction impulses 

 and as a result the sending and receiving brushes may depart con- 

 siderably from their normal phase position. This would cause errors 

 to occur in the first signals received upon resumption of transmission if 

 means were not taken to bring the brushes back into proper phase 

 relationship before the transmission of actual signals was begun. 

 This is provided for by arranging to have the distributors transmit, 

 at the close of each switching period, a number of "spacing" signals 

 which do not alTect the receiving printers since they are not connected 

 in circuit until a sufficient number of reversals have occurred in the 

 line current to correct the receiving brush into the proper position. 

 The transmission of signals which must be recorded by the printer is 

 then started. 



As the length of the interval allowed for these switching operations 

 is determined by a definite number of revolutions of the distributor 

 shaft, which may be set to rotate at various speeds, the gearing between 

 the distributor shaft and the timing mechanism is designed to allow 

 for a five-second switching period when the distributor is rotating at a 

 speed which corresponds to the maximum transmission speed of the 

 circuit. 



Although this system lacks the advantage of absolutely continuous 

 communication in both directions, it possesses another feature which 

 goes far toward offsetting, if it does not entirely outweigh, the ad- 

 vantages afforded by the duplex method. Almost all of the long cables 

 of the world run in an east and west direction and the difference in 

 time between the terminal stations of those cables results in an unequal 

 distribution of traffic in the two directions except perhaps during a 

 comparatively short time each day. The provision of the selective 

 timing mechanism permits the total traffic capacity of the cable to be 

 divided between eastward and westward transmission in about the 

 same proportion as the eastward traffic load bears to the westward 

 load and thus permits efficient utilization of the entire traffic carrying 

 capacity of the cable. 



The Synchronous Vibrating Relay 



The vibrating relay principle was first suggested by Gulstad •''• who 



applied it to short cables for overcoming the effects of distortion. 



As originally used, it consisted of a sensitive polarized relay provided 



with a line winding, upon which the received signals were impressed, 



and two auxiliary windings included in a local vibrating circuit 



adjusted to cause the relay armature to vibrate continuously when 



«K. Gulstad, "Vibrating Cable Relay," Elec. Rev., London, Vol. 42, 1898; Vol. 

 51, 1902. 



