454 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



is generated in the secondary winding of each transformer when the 

 flux is changed from saturation at one polarity to saturation at the other 

 polarity. The flux genei'uted in the core of each transformer depends 

 upon the number of turns in the two primary windings and upon the 

 current flowing in each winding. In order to assure that all pulses be 

 substantially alike as to wave form and amplitude it is necessary that 

 the total miximum ampere-turns on each core be ec^ual. In order to 

 cause each transformer to generate a pulse at a suitable time during 

 each half-cycle of the excitation current the total ampere-turns driving 

 flux through the transformer cores must be controlled so that the flux 

 in each transformer is zero at the time assigned to the pulse which that 

 transformer serves to generate. These conditions determine the number 

 of turns and the polarity of each winding when the angular position of 

 the desired pulse is fixed in relation to each half-cycle of the basic ex- 

 citation current. 



Since the magnetic flux in each transformer is reduced to zero two 

 times during each cycle of excitation current, it follows that a combina- 

 tion of two pulses representing a digit must occur during each half-cycle 

 of the excitation current and that each combination of two pulses rep- 

 resenting a digit is of opposite polarity to the preceeding two pulses. 

 The capacitor through which the pulse generating transformer secon- 

 dary windings are connected to the line is so proportioned to the im- 

 pedances of these windings and to the impedance of the line that each 

 half-cycle pulse as generated by a transformer is applied to the line as a 

 single complete cycle of alternating current of about 1 millisecond 

 duration. 



A selector switch, which is the internal mechanism connected with 

 the finger wheels pre-set by the subscriber, serves to interconnect the 

 transformer pulse windings with the line through the stepper switch. 

 Thus, pulses representing any of the digits to 9 may be impressed 

 across the telephone line as any desired part of a complete telephone 

 number in accordance with the setting of the selector switch. 



The stepper switch employs ten relays of the glass-sealed dry-reed 

 type and each of the relays has an individual coil surrounding two nor- 

 mally open reed contacts. The reeds are polarized by a permanent mag- 

 net of sufficient strength to hold the reed contacts closed but not strong 

 enough to close them until assisted by current of the correct polarity 

 through the winding. A reverse current through the winding is required 

 to release the contacts. In addition a common winding is provided which 

 surrounds all of the reeds in such a manner that when a current of suffi- 

 cient magnitude is passed through the winding the reeds of a predeter- 



