762 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



(25r i graded series of non-adjustable artificial lines using lumped shunt 

 condensers and lumped series inductances, these ''lumps" being electrically 

 smaU enough to avoid objectionable "lumpiness" effects. The required 

 building-out units are connected in tandem with the loaded cable pair under 

 adjustment. 



The adjustable unit consists of a single-layer, bifilar winding on a non- 

 magnetic spool about 4 inches long and f inch in diameter. The wiring of 

 the unit to its two pairs of Une terAiinals is such as to provide a series in- 

 ductive-aiding connection of its fine windings when the unit is serially 

 inserted in the cable pair according to plan. The diameter of the spool is 

 chosen to provide the desired ratio of inductance to capacitance per bifilar 

 turn, taking into account the increase of capacitance of the winding which 

 is caused by a wax-dipping process. 



The inductance coils used in the non-adjustable units are very small air- 

 core solenoids. Their inductances are adjusted (in manufacture) to have the 

 correct electrical relations with the associated very small, mica-type, shunt 

 condensers. 



The complete building-out adjustment for an individual loading section 

 usually involves the use of one or more artificial-line units in tandem with 

 an adjustable unit. The adjustments are made in terms of capacitance 

 measurements, since this procedure automatically provides the required 

 series inductance. Capacitance measurements of the cable pair to be ad- 

 justed and of preselected non-adjustable units precede the precision adjust- 

 ment of the wire-wound unit. This latter is accomplished by removing an 

 integral number of bifilar turns from the winding, to meet the capacitance 

 requirements, after which the shortened winding is reconnected to its main 

 line terminals. 



Housing of Building-Out Units: For flexibility in installation, the different 

 electrical sizes of building-out units are "potted" in individual containers 

 of the sam3 size. These are much too large for installation in the loading 

 splice-sleeves. Accordingly, in the cable- type cases for full-weight loading 

 coils, and in the open-wire terminal pole cases for terminal loading units, 

 space is provided in compartments with removable covers for the installation 

 of the cable building-out units. The connections to the main cable circuit 

 are made to terminal strips mounted in these compartments. Thus the 

 installation of the building-out units can be made after the loading coils and 

 loading units have been spliced to the disc-insulated incidental cables. The 

 terminal loading units used at office ends of loaded entrance cables also 

 include space and wiring provision for the installation of building-out units 

 which may be required on the cable side of the terminal loading unit. 



