416 BELL SYSTEM TECHNICAL JOURNAL 



between the brushes on the distributors at the two ends of the cable 

 is only about one and one half degrees of revolution and in order to 

 hold within this limit it was necessary to design a driving unit in 

 which the phase shift, resulting from variations in the line voltage, 

 was reduced to a minimum, and to arrange the gearing and coupling 

 between the driving motor shafts and the various rotating brush 

 arms so as to reduce to a minimum any lost motion or back lash. 

 The driving unit consists of two motors: the one which supplies the 

 power for driving the brushes is a dynamotor in which the DC side 

 is used as a motor to supply the power and the AC side is included in 

 a circuit with an electrically driven tuning fork which controls the 

 motor speed within very close limits; and the other is a phonic wheel or 

 La Cour motor driven from the same driving fork. This motor normally 

 supplies little if any power for driving the distributor but by increasing 

 or decreasing the load it prevents the occurrence of any appreciable 

 phase shift in the DC motor due to variations in the driving voltage. 

 In order to prevent slight shifting in the phase of the brushes due to 

 vibration and axial twisting in the shafts and gears, it was necessary 

 to employ much heavier construction in the rotating parts than is 

 actually required to transmit the small amount of power used. The 

 cutting of the gears, the distributor segments, and timing cams was 

 done with the utmost precision to eliminate mechanical errors. The 

 distributor segments included in the vibrating relay circuit are heavily 

 faced with coin silver to reduce variation in the resistance of the 

 contact between them and the rotating brushes. 



The satisfactory operation of the system depends upon the accuracy 

 with which the various relays in the system follow and repeat the 

 signals. None of the available types of relays were found to be 

 sufficiently reliable to permit of use in the system and it became 

 necessary to develop for the purpose a new type of high speed relay 

 shown in Fig. 5. The size and inertia of the parts comprising the 

 moving system of this relay were reduced as much as possible in order 

 to secure quick response and freedom from contact chatter at the 

 highest operating speeds. A magnetic circuit was designed in which 

 the efTects of magnetic hysteresis are practically negligible, which 

 results in the relay always operating upon the same value of current 

 irrespective of its previous magnetic history. Permanency of adjust- 

 ment, which is essential in relays used in this class of service, was 

 obtained by adhering to standards of accuracy and precision of 

 manufacture heretofore considered unnecessary in relay construction. 

 The accuracy with which relays of the new type will operate at high 

 speeds and the entire freedom from contact chatter is illustrated in 



