120 BELL SYSTEM TECHNICAL JOURNAL 



types of offices, and which may not be needed with "through line" 

 repeaters in some cases, will be furnished as a separate panel from the 

 basic repeater unit. The apparatus which will permit the repeater 

 to be used for cord circuit operation is also to be furnished as a sep- 

 arate unit and may be used in place of the through signaling unit, 

 without changing the basic repeater. The filter which will determine 

 the cut-off frequency of the repeater will also be furnished as a separate 

 piece of apparatus mounted on the repeater set, thus the repeater 

 may be suited to any desired type of line by providing the proper 

 filter. 



Another interesting phase of repeater development has been that in 

 connection with the power supply for the vacuum tubes. This in- 

 cludes (1) a source of filament current (2) a source of plate potential 

 and (3) a source of grid potential. In meeting the requirements of 

 cable installations the principal improvements desired have included 

 the use of batteries common to as many repeaters as possible, in place 

 of individual batteries, closer regulation of potentials and the elimina- 

 tion of dry cells where practicable. 



In some of the earliest installations a separate six-volt storage 

 battery was used to supply the filament current for the vacuum tubes 

 of each repeater set. Later, the filament current supply was taken 

 from an 11-cell central office storage battery through a rheostat. 

 As the potential of the 11-cell central office battery, normally 24 

 volts, varied from 20 to 28 volts during the operation of a charge and 

 discharge routine, it was necessary to adjust the rheostat at frequent 

 intervals to maintain constant current in the vacuum tube filaments. 

 With the greatly increased number of repeaters per station which has 

 occurred in cable systems, the maintenance involved in readjusting 

 the filament currents would have become prohibitive on this basis. 

 Accordingly, for the larger installations, duplicate 11-cell batteries 

 normally floated from generators and provided with an emergency cell 

 to maintain voltage during an emergency discharge are proposed. 

 By this improved arrangement it is expected to be possible to maintain 

 the filament voltage within one volt up or down from its normal 

 value, even during an emergency discharge, until the batteries are 

 almost completely discharged. This improved regulation will en- 

 tirely eliminate adjustments of the individual repeaters during opera- 

 tion to secure proper values of filament current. 



For the plate voltage supply dry cells have sometimes* been used 

 in small installations. These are now being displaced to a large 

 extent, by small storage cells, two groups being used so that one 

 group may be charged while the other is in service. In the large cable 



