SUBMARINE TELEGRAPH CABLES 431 



corrections. It must also be able to operate satisfactorily with signals 

 in which the weaker components may be as strong as 100 millivolts. 

 An output of about fifteen milliamperes at 15 volts should be available 

 and this output must be adjustable by small steps. It must be 

 capable of handling currents containing frequencies between a small 

 fraction of a cycle and about 180 cycles, the particular part of this 

 band of frequencies which is utilized depending on the nature of the 

 cable and the speed at which it is operated. 



These requirements are met in the present cable amplifier/^ by 

 circuits which are shown in the upper half of Figure 1. The amplifier 

 proper consists of four stages of vacuum tubes, the first three being de- 

 signed for high voltage-amplification and the last for large current out- 

 put. An additional output stage is provided for the purpose of increas- 

 ing the flexibility of the amplifier by permitting two separate classes of 

 apparatus to be operated simultaneously. The coupling between 

 stages is a combination of two types, the coupling for the very low 

 frequencies being through a resistance capacity network while that 

 for the higher frequencies is by means of an auto-transformer of special 

 design or by highly damped resistance, inductance and capacity net- 

 works. The amplifier is connected to the cable through an input 

 network and a shielded transformer. The input network assists in 

 shaping the signal and prevents the first stage of the amplifier from 

 being overloaded by the strong low frequency components of the 

 signal arriving in the cable. The transformer permits earthing the 

 filaments of the amplifier tubes and their associated batteries and 

 avoids the short circuiting of the long balanced sea earth. The latter 

 is used to reduce the effect of electrical disturbances on that part of 

 the cable which lies in shallow water near the shore.^" The require- 

 ments of this transformer are quite unusual as it must have a satis- 

 factory voltage regulation from .2 to 200 cycles per second. 



The ability to operate on voltages which may vary widely from 

 time to time makes it necessary to provide a suitable range of adjust- 

 ment of amplification. This is accomplished by providing that the 

 secondary windings of the input transformer may be connected in 

 series or in parallel and the plate coupling resistances of the tubes 

 varied by a factor of four to one. A potentiometer placed between 

 the second and third stages of the amplifier allows a variation of 

 twenty to one in the voltage transmitted to the third stage, and 

 with other adjustments as mentioned above, increases the total range 



^A. M. Curtis, U. S. Patents Nos. 1,586,970 and 1,586,972, June 1, 1926, and 

 1,624,395 and 1,624,396, April 12, 1927. 



12 J. J. Gilbert, Bell System Technical Journal, July 1926; also British Patent No. 

 218,261, August 31, 1925, and Canadian Patent No. 265,944, Nov. 16, 1926. 



