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BELL SYSTEM TECHNICAL JOURNAL 



are windings of a transformer with a mid tap. If a voltage 2Eq 

 having a frequency of 720 cycles (which is the frequency corresponding 

 to the desired speed of 1,200 R.P.M.) is supplied to the bridge it will 

 be apparent that the output voltage £2 will be zero. If, however, 

 the speed is low the tuned circuit will have a condensive reactance 

 while if the speed is high it will have an inductive reactance. The 

 output voltage £2 will, therefore, change abruptly 180 electrical 

 degrees from a speed below 1,200 to a speed above 1,200. This 

 characteristic is shown in Fig. 4. 



The use of the above described bridge circuit gives a very sharp 

 characteristic due to the fact that the effective resistance component 

 of the tuned circuit is balanced out by the adjacent resistance arm of 



PHASE 



Fig. 4 — Output voltage characteristic of bridge circuit. 



the bridge. In this way an overall characteristic of the desired 

 sharpness is secured using a comparatively small and inexpensive 

 coil and condenser. 



Fig. 5 shows the complete control circuit. The output from the 

 bridge circuit is supplied to the grid of tube F4 which is called the 

 detector tube. The plate voltage of this tube comes from the 720- 

 cycle generator through the step-up transformer T^. The phase of 

 this voltage, therefore, remains constant. The phase angle of the grid 

 voltage, however, comes from the bridge output circuit through the 

 step-up transformer Tz and as previously explained suffers a sudden 

 reversal of phase as the speed passes through 1,200 R.P.M. Fig. 6 

 shows the resulting current characteristic through tube F4. This 

 current flows through coupling resistance Ri which drives the grids 



