AUTOMATIC SWITCHING SYSTEM 453 



digits of successive magnitudes differ by 0.4 milliseconds. Thus, digit 

 1 is represented by a start pulse followed l\v a stop pul.se 3.2 milliseconds 

 later; digit 2 is represented by a start pulse followed by a stop pulse 3.6 

 niilli.seconds later; and so on. It will l)e observed that the stop pulse for 

 the digit is 6.8 milhseconds after its start pulse and 3.2 milliseconds 

 before the next succeeding start pulse. Thus, there is provided an in- 

 crement of time of 3.2 milliseconds for the decay of the start pulse, 

 increments of 0.4 milliseconds each for the generation of a pulse at any 

 one of the ten times necessary to represent the various digits, and a 

 last increment of 3.2 milliseconds to permit a stop pulse to decay should 

 it occur at the end of the ninth increment of time. 



Referring again to Figure 6, the signaling pulses are generated by the 

 eleven pulse transformers shown. These saturation-type transformers 

 are assigned, one for each of the numerals to 9 and one for the start 

 pulse. The excitation for the signaling apparatus is a constant amplitude 

 50-cycle current of sinusoidal wave form transmitted from the central 

 office on a simplex circuit consisting of the two fine wires to the set with 

 ground return.* The currents from the line wires pass into the signaling 

 apparatus through the windings of the latch magnet. These latch mag- 

 net windings thus serve also as a simplexing coil and since the excitation 

 magneto-motive-forces in the two windings are mutually opposing there 

 is no reaction on the latch itself. 



From the simplex coil the excitation current flows through a stepper 

 switch and its shunting phase shifter to a phase splitting network in 

 which the current is converted to a two phase source with its two cur- 

 rents 90 degrees out of phase. Each of the pulse generating transformers 

 has a single secondary and two primary windings. The primary windings 

 of the transformers are serially interconnected and connected with the 

 two phases of the excitation current so that one phase is applied to one 

 primarj^ winding of each transformer and so that the other phase is 

 applied to the other primary winding of each transformer. The secondary 

 windings are connected across the line through the pre-set selector, con- 

 tacts of the stepping switch and a series capacitor. The secondary wind- 

 ing of the pulse transformer for the start pulse is in a lead common to 

 all the stop pulse secondaries. 



The magnetic core of each pulse transformer is designed to be satu- 

 rated except for very small values of ampere-turns, and a voltage pulse 



* The time interval spacings of signal pulses given in this section and in the 

 following section on the signal receiver are based on a SO-cj^cle control current. 

 The system operated satisfactorily on 50 cycles. However, in most of the labora- 

 tory tests a control current of 45 cycles per second was used since a stable source 

 of this frequency is readily derived from commercial 60-cycle power sources. 



