ADVANCES IN CARRIER TELEGRAPH TRANSMISSION 193 



such discharge to occur at the same moment and be of equal magnitude 

 from both wires of the pair to ground whenever it does operate. 



Extensive tests and practical experience have shown that the above 

 arrangements are quite effective, affording a reduction of about 95 



TOLL 



ENTRANCE 



CABLE 



CABLE POLE OPEN-WIRE LINE CABLE POLE 



I CENTRAL 

 -r- OFFICE 

 GROUND 



CENTRAL OFFICE 



PROTECTOR 

 BLOCKS 



CENTRAL OFFICE 



r^ — 1 



TO 

 SWITCH- 

 BOARD 



Fig. 18 — Drainage arrangements. A. Direct drainage. B. Protector drainage. 



per cent in the number of disturbances occurring on the line as well as 

 in the number of errors in transmission resulting therefrom. 



Carrier Supply 



The inductor-alternator source for carrier frequencies used in the 

 original installation has been retained except for the addition of two 

 channel frequencies, and the substitution of improved means of speed 

 (frequency) control. The first mechanical governor was soon super- 

 seded by a center-contact device which is much less erratic in operation. 



The carrier frequency can be shifted over quite a few cycles from its 

 correct value without affecting signal distortion appreciably but rapid 

 speed variations have a more serious effect. Two methods have been 

 used to overcome this difficulty. The first consists effectively in 

 making the generator part of a system having great overall inertia, the 

 second in subjecting it to very rigid control. 



To secure the first end the VF generator is driven by a synchronous 

 motor operated from the lighting circuit. This arrangement can, of 

 course, be used only where the frequency of the commercial power is 

 regulated within narrow limits. The required generator speed being 



