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number of positions. The unit containing the indicating meters and 

 associated controls for mounting at the telegraph board is illustrated 

 in Fig. 3. The set is also provided in portable form for temporary 

 use in cases where a permanent installation is not justified. 



B. Telegraph Stability Test Set 

 Recording meters have been used for a number of years in the 

 measurement of the transmission stability of telegraph circuits.^ In 

 such a measurement a continuous graphic record is made over as long a 

 period as desired of the variations in the bias and of the number and 

 time of occurrence of fortuitous effects which would impair telegraph 

 service. For this purpose telegraph reversals are impressed at the 

 sending end and a recorder at the receiving end makes a record of the 

 bias of these reversals. This type of test was found to be of such 

 utility in field work that standard stability test sets were produced 

 for this purpose. 



* GALVANOMETER RELAY 

 ^(* RECORDING METER 



Fig. 8 — Schematic circuit of telegraph stability test set. 



The circuit of the telegraph stability test set is indicated in Fig. 8. 

 This is essentially a simple bias-measuring circuit combined with an 

 alarm circuit to indicate when given values of bias are exceeded. 

 Movement of the armature of loop relay A from M to S contact or 

 vice versa in response to the received signals causes a battery E to be 

 connected first to one arm (resistances 6 and 4) and then to the other 



