248 BELL SYSTEM TECHNICAL JOURNAL 



doing reduces the peak voltage which is applied by the signals to the 

 primary of the transformer T and also the peak voltage which is 

 applied to the grid of the first vacuum tube. By insertion of this 

 shaping network between the cable and the transformer, overloading 

 and consequent distortion are prevented. 



The transformer T permits insulating the amplifier and its batteries 

 from the cable and thereby allows the amplifier to be connected 

 directly to earth ^ and to be effectively shielded from local electrical 

 disturbances. Without the transformer or other means to insulate 

 the amplifier from the cable it would be impossible to use an earthed 

 amplifier and at the same time to secure the advantage of the balanced 

 sea-earth in eliminating interference. The requirements for this 

 transformer are very severe since it must be effective for frequencies 

 as low as 0.2 cycle per second and at the same time must be con- 

 structed so that it will not pick up the external electrical disturbances 

 generally prevalent in cable stations. The use of a permalloy core 

 and a permalloy shield has made it possible to meet these requirements 

 in an instrument occupying less than one third of a cubic foot. 



Connected between the successive stages of the amplifier are the 

 signal-shaping networks A^3, Ni and N^. These networks serve both 

 to adjust the shape of the signal and to reduce the efifects of inter- 

 ference outside of the signal range. Considerable advantage is 

 gained from the fact that there are in the entire system five signal- 

 shaping networks, each separated from its neighbors by either the 

 cable or the vacuum tubes. This arrangement permits independent 

 adjustment of the separate networks with very little interaction 

 between them and greatly facilitates the systematic correction of 

 signal shape. 



The values of the various resistances, inductances and capacities 



in the networks at the receiving end depend, of course, on the cable 



as well as on the type of telegraph apparatus employed; for this 



reason most of the important circuit elements are made adjustable. 



The adjustments are made by trial, but in spite of the apparent 



complexity of the networks, which are more elaborate than would 



be required for any given cable with fixed operating requirements, 



the adjustments necessary to adapt the apparatus to any particular 



conditions can be made quite systematically. After the shaping 



adjustments required for a particular cable have been worked out, 



which usually takes not more than a few days, the amplifier can be 



adjusted for any speed in the range of the cable in a few minutes. 



»The earth connection for the amplifier is preferably made to a short " sea-earth" 

 conductor terminated on the cable sheath at a few miles from shore. The same 

 earth conductor may be used for a transmitting earth. 



