Masters of the Test Board

Open Wire Test Board, c. 1989.


Masters of the Test Board

Wheatstone Bridge and other features.

Balanced vs. unbalanced telephone circuit.

Hand drawn early metalic telephone system.

Hand drawn “Hook & Eye Link” system.

Hand drawn phantom circuit configuration.


Efficient Telephone Operation: The Open Wire System


It has been commented, that “open wire was a pretty good–to excellent–speech transmission medium, if you observed a few precautions.”  It was with that caveat, that open wire progressed and mastered the telegraph and telephone transmission medium since 1843.  


To effectively obviate any limiting factors, and to prepare as sleek and seamless an operation as open wire could effectively contribute to the toll and exchange outside telephone plant, was to engage the Test Board Operators or Clerks to the best of their abilities.  These people montiored the open wire plant’s effective speech, telegraph, symbols, information and radio programming on a day-to-day, if not hour-to-hour basis and to locate trouble as it occurred and correct it before outages might occur.


Limits Affecting Telephone and Telegraph Transmission


Every open wire line operates within strictly defined limits, taking into consideration a number of factors impacting on the line’s continued effectiveness to transmit speech or data.   Here are some basic facts which designers of open wire (and multi-pair cable) must appreciate when planning such large capital investments:


Distortionless speech along the wire’s path from C. O. to subscriber and returned to C. O.

Speech volume and quality must be intelligiable.

Losses must be limited as far as possible.  Such as a) Resistance; b) Inductance; c) Electrostatic capacity; d) Leakage

Losses encountered at the Central Office, such as inside plant negative electrical values.

Inductive interference which takes the form of “crosstalk” and “noise”.

Lines which are not balanced.

Continuity of service.

Comprehending the potential for growth and properly planning facilities for future economic growth.

Special circuit design: different classes of service require different grades of transmission.

Highest degree of equipment, hardware and facilities service standards to meet the economics of profitable return on the investment.