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BELL SYSTEM TECHNICAL JOURNAL 



the other group of windings to operate the relays to their spacing 

 contacts. The series-parallel arrangement of the relay windings shown 

 in the figure has the advantages that with the same number of relays 

 in series and in parallel the combined inductance is only that of a 

 single relay, and that any relay may be removed for inspection without 

 materially affecting the operation of the others. Only one transmitting 

 battery is used in this circuit and thus errors due to battery inequalities 

 are avoided. As indicated in the figure, a spark-reducing circuit is 

 associated with each output for the purpose of minimizing arcing and 

 to neutralize the effect of travel-time of the relay armature which 

 would otherwise cause the transmitted signals to be biased to spacing 

 when opening and closing the circuit under test. 



Each group contains 6 relays in parallel and any number of groups 

 up to 8 may be used in series to provide a maximum of 48 outputs 

 to meet the requirements for offices of various sizes. 



The above-mentioned sources of signals as maintained in the field 

 are usually accurate to within a few per cent distortion. For special 

 uses it is possible to reduce this inaccuracy somewhat by additional 

 maintenance. 



Distorted Test Signals ' 



A repeating device has been provided, primarily for transmission 

 maintenance, by means of which the distortion of telegraph signals may 

 be increased by predetermined amounts. The set is generally used 

 with a source of undistorted signals to provide test signals having 

 known amounts of distortion. 



BIAS-SWITCHING 



RELAY f60 OPERATIONS 



PER MINUTE) 



Fig. 18 — Bias-producing circuit. 



The schematic circuit of the signal-distorting device is shown by 

 Fig. 18. The signals to be distorted are connected to the input and 

 are repeated by the receiving relay (Rec) of the device into a biasing 



