TELEPHONE TRANSMISSION OVER LUNG CABLE CIRCUITS 91 



limits will first be described, after which the automatic transmission 

 regulators will be discussed. 



VIII. Important Tests and Adjustments 



In order to hold the repeater gains constant, close inspection limits 

 are placed on the vacuum tubes during the course of manufacture to 

 insure great uniformity of the product, as well as consistency of per- 

 formance. In operating the repeaters, considerable care is taken to 

 maintain constancy of the operating currents and voltages. The 

 operating limits of currents and potentials together with the cor- 

 responding gain variations for one of the types of tube in common 

 use are given in the following table: 



In addition to maintaining the tube currents and voltages within 

 the required limits, the gains of the individual repeaters are checked 

 periodically. Suitable adjustments are made when the repeater gains 

 fall outside of the prescribed limits. When the filament emission of 

 a tube becomes so low that the above specified variation in the fila- 

 ment current results in more than 1 mile gain variation the tube is 

 replaced. 



f_Q_jr- 



Fig. 14 — Device for measuring telephone repeater gains. 



A gain measuring device as indicated schematically in Figure 14 

 is employed for this purpose. The measurement of gain is effected 

 by comparison of the voltages across two resistances, one of which 



