SUBMARINE TELEGRAPH CABLES 433 



used with the mechanical ampHfying systems on low speed cables; 

 they usually supply directly to the relay a "zero correcting" current 

 which depends upon the past history of the signal. ^^ 



When a vacuum tube amplifier is employed it is more convenient 

 to apply the zero correction to the grid of the last stage vacuum tube 

 as this results in the most economical utilization of the correcting 

 battery and its circuits. The zero correcting apparatus is mounted 

 in a cabinet adjacent to the amplifier and differs considerably in 

 principle from that hitherto used with mechanical amplifi.ers. 



The three element moving armature polarized relay, which had 

 been designed for use in loaded cable operation generally, was changed 

 in some details and adapted for use in the zero corrector. It is 

 capable of operating at a high speed and also discriminates very 

 accurately between currents of slightly differing values. When 

 actuated by the normal signal its armature contacts vibrate between 

 the fixed contacts, not touching either unless the zero of the signal 

 deviates more than about three per cent from its proper position. 

 When this deviation does occur the relay contacts close the circuit 

 for an instant at the peak of a signal wave, and permit the battery 

 to which they are connected to charge a condenser through a com- 

 paratively low resistance. The charge on this condenser then passes 

 gradually to a second condenser through a high resistance and at the 

 same time commences to be discharged from the second condenser 

 by a shunt resistance. The voltage on this second condenser is 

 applied to the grid of the last stage of the amplifier in such a sense that 

 it produces a deflection of the amplifier zero in the direction opposite 

 to the deflection which caused the relay contacts to come together. 

 This correcting voltage is applied at a rate which is slow enough to 

 prevent it from distorting the signal and the rate at which it is dissi- 

 pated by the shunt if no further contacts take place is still slower. 

 It should be noted that these rates of charge and discharge, while 

 adjustable, need not bear any accurate relation to the shape of the 

 signal itself. The correction is usually applied rapidly enough so that 

 the zero is brought back to normal within the duration of about five 

 signal pulses and the proper operation of the circuit prevents the zero 

 from passing beyond limits about five per cent of the signal amplitude 

 either side of the normal position. A somewhat simplified circuit of 

 the relay control unit which includes the zero corrector is shown in 

 the lower part of Fig. 1. 



^' A system in common use is due to S. G. Brown (British Patent No. 6,275, 

 February 20, 1913). Other systems have been invented by D. K. Gannett and M. 

 Kirkwood, U. S. Patent No. 1,548,878, Aug. 11, 1925, and R. C. Mathes, U. S. 

 Patent No. 1,295,553, February 25, 1919. 



