JOINT DE VELOPMENT A ND LOW-FREQ UENC Y IND UCTIO N 22 1 



the relays at a protector point are electrically interlocked, so that 

 when any relay operates all line wires are grounded within a few cycles. 



Several trial installations of relay protectors have been made and 

 are under observation. To guard against voltages to ground within 

 the exposure these protectors have to be placed within, as well as at 

 the ends of, the exposed section of line. Where the longitudinal 

 induced voltage is large, protectors are required at a number of points 

 within the exposed section. 



The effective application of such protectors requires grounds of the 

 order of one or two ohms and an important feature of the investigation 

 is to devise methods of constructing and maintaining such grounds at 

 remote points along the line. 



The subcommittee is investigating in the field and in the laboratory 

 the effectiveness, cost, reaction on service, and other practical ques- 

 tions relating to the installation and maintenance of this method of 

 protection. 



Acoustic Shock Reducers. — Since acoustic shock due to induced vol- 

 tages involves dissymmetrical discharges across the two sides of the 

 protector, efforts have been made to devise a protector which would 

 break down and discharge symmetrically, i.e., provide two reliable 

 low-impedance paths for heavy discharges, which would at all times 

 have very closely the same arcing impedance. Thus far the subcom- 

 mittee has not been successful in developing a practicable protector of 

 this kind. 



For the purpose of equalizing the voltages on the protector during 

 the discharge period, an accessory device termed a "discharge balance 

 coil" is under investigation. It consists of two equal windings on a 

 common core, each in series with the discharge gap of one side of the 

 line, and so arranged that the fluxes set up by the circuits in the two 

 windings are in opposition. The "booster" action of this coil tends to 

 equalize the discharge currents. This reduces acoustic shock from 

 induced voltages, provided all protectors are so equipped and the line 

 itself has no large unbalances. When however, voltage is impressed 

 on one wire only of a telephone circuit, as by accidental contact, these 

 coils have a detrimental effect on the action of the protector in reducing 

 voltage to ground, as they introduce impedance in the protector dis- 

 charge path. 



Development work is also being conducted on other types of acoustic 

 shock reducing measures which do not attempt to prevent unbalanced 

 current but merely to shunt it out of the telephone receiving circuit. 

 Obviously a device acting on this principle to be successful must be 

 practically instantaneous in operation. One of the most promising of 



