196 BELL SYSTEM TECHNICAL JOURNAL 



tend to move one or the other relay armature to the opposite contact 

 according as the frequency is high or low, and thereby ring the alarm. 

 This resulting current also indicates the amount by which the fre- 

 quency departs from its nominal value. 



The frequency indicator may be used with either the mechanical 

 governor or with the synchronous drive, but it is of little use with the 

 vacuum tube tuned circuit control as the latter is too precise to 

 register any indications. 



The frequency indicator does not permit a very close adjustment of 

 the mechanical governor nor does it provide a satisfactory check for 

 the correctness of the frequency of the commercial power when a 

 synchronous motor is used, so a stroboscopic method has been adopted 

 as an ultimate standard of comparison. This stroboscope consists of a 

 cylindrical target made up of three distinct peripheral rows of alternate 

 black and white segments mounted on the end of the generator shaft. 

 These segments may be viewed by means of a tuning fork fitted with 

 overlapping metal plates attached to the ends of the tines. Slits cut 

 in these plates lie opposite each other when the fork is at rest. When 

 it is set vibrating, vision through the slits can therefore be established 

 momentarily twice during each complete oscillation. By illuminating 

 the target with a steady source of light the apparent direction of motion 

 of the dots can thus be observed by looking through the slits. The 

 middle row of segments on the target is so proportioned as to appear 

 stationary if the speed is practically correct while the outer rows appear 

 respectively stationary if the speed is approximately 1 per cent above 

 or below the nominal value. 



For offices where the frequency of the commercial supply is suffi- 

 ciently stable, an additional and somewhat more convenient method 

 for checking the speed has been made available. It consists of a 

 special target mounted on the generator shaft, which is illuminated by 

 a neon lamp associated with a wave-shaping circuit which makes the 

 flashing time a very brief portion of each pulse of the 60-cycle current. 

 In other words, the attendant in this case looks at the target constantly 

 under intermittent illumination, while in the former case he views it 

 intermittently under constant illumination. 



Testing Facilities 

 While developments in carrier telegraph equipment have resulted in 

 considerable economies, the ever increasing demand for service which 

 is freer from errors and interruptions and is adaptable to circuits of 

 greater length and complexity has tended to render the maintenance 

 problem increasingly difficult and time consuming. Voice-frequency 



