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BELL SYSTEM TECHNICAL JOURNAL 



and the resistance of the ground connections, reaching optimum values 

 of over 90 oer cent. Table C gives the magnitude of this shielding 

 for various selected sizes and lengths of cable. Table C brings out 

 distinctly the variation in the magnetic shielding due to the factors 

 mentioned above. The effect of cable sheath shielding in several 

 typical cases is further indicated in the Appendix. 



TABLE D 



Relative Susceptiveness of Several Types of Station Sets 



Noise in Receiver Branch for 100 



Noise Units to Ground — 



Average Power Wave Shape 



{One Station on Line-Effect of Set 



Only) 



General Description of Station Set 



350 Noise Units Approximately 



120 Noise Units Approximately 

 120 Noise Units Approximately 



Class 1 — Types of sets in most common use 

 today 



a. Sidetone type of party-line set using 



8A ringers or equivalents (one end 

 of ringer grounded). (Common- 

 battery talking and signaling) 



b. Same type — -local-battery talking 



c. Magneto party-line set (52A Ringer 



or equivalent) 



d. Individual-line set — any type 



Class 2 — Types of sets frequently encountered 



a. Sidetone type of 4-party full-selective 



or 8-party semi-selective set (using 

 relay or cathode tube to connect 

 ringer to circuit during ringing 

 period) 



b. Four-party selective or 8-party semi- 



selective sets employing high im- 

 pedance ringers or relays con- 

 nected to ground 



c. Eight-party selective (harmonic ring- 



ing) sets employing ringers con- 

 nected to ground and tuned to 4 

 different ringing frequencies 



d. Ground-return rural circuits (usually 



of magneto type and having code 

 ringing) 



Class 3 — Special types of sets 



a. Sidetone type of party-line set using 



split-condenser and higher im- 

 pedance ringer (one end of ringer 

 grounded) 



b. Type of party-line set using split con- 



denser arrangement with 8A 

 ringer or equivalents (one end of 

 ringer grounded). 



(c) Station Apparatus 



Individual-line stations employ a "bridged" ringer connection, i.e., 

 the ringer is, in effect, connected between the two line conductors. 



Negligible 

 Negligible 



About 30 



Limited data indicate that, de- 

 pending on frequency for which 

 ringer is tuned, noise will range 

 from about 100 to about 400 

 units 



3500 or more noise units 



About 20 noise units 



About 90 noise units 



