TRENDS IN PRACTISE AS AFFECTING COORDINATION 173 



stitution of copper for short tributary toll circuits. Coordination of 

 the iron wire circuits is relatively difficult because of the development 

 of resistance unbalances at the wire joints. The transmission efficiency 

 is also improved by the reduced resistance afforded by the copper but 

 this effect is generally of secondary importance in the short tributary 

 circuits. 



In toll offices improvements have been made in the balance of coils 

 and condensers used for superposing telegraph on the telephone circuits. 

 The use of repeating coils, commonly used for side-circuits, has been 

 extended to phantom toll circuits. These coils act as insulating trans- 

 formers to prevent noise voltages from the outside conductors being 

 impressed upon the intricate cabling and equipment of the office. 



Referring to the local plant, there are several noteworthy examples 

 of modifications made principally for the purpose of reducing suscep- 

 tiveness. Investigation such as that of the coordination between 

 power and telephone distribution plants conducted at Minneapolis 

 by the Joint General Committee, stimulated the development of means 

 for reducing the susceptiveness of the telephone distribution plant. 

 Present practises call for the interconnection of aerial and underground 

 cable sheaths and the grounding of the aerial sheath in order that the 

 benefits of the shielding action of the sheath currents as previously 

 described, may be realized for noise induction. In cases where elec- 

 trolysis conditions do not permit direct grounding, condensers of the 

 electrolytic type are employed to prevent the flow of direct currents. 

 The telephone circuits have long been equipped with over-voltage 

 protectors for the purpose of protecting apparatus and cables against 

 lightning waves and against power frequency transients from the lower 

 voltage distribution circuits, also with fuses for opening the lines in 

 cases in which heavy currents flow. The trend in development of these 

 devices has been principally toward more uniform operation and lower 

 maintenance costs. With the rapid increase of voltage and capacity 

 of power circuits generally, experimental studies have been undertaken 

 of further means for maintaining the safety of persons working on or 

 listening on the telephone circuits. At the present time, development 

 work is being done on various devices for this purpose, some of which 

 are fundamentally different in design and operation from those pre- 

 viously used. It is hoped that these devices, which are discussed in 

 one of the following papers, will afford increased protection against 

 overvoltages and improve coordination conditions. 



Trends in Power System 

 In the field of power generation marked attention has been paid, 

 from the start, to methods of improving the efficiency of the generating 



