Today we have a paper by L.E.Trump posted to the Telegraph Technology section of the
Telegraph Lore website.
Enjoy
73
KJ
Testing on Open Wire Telegraph Lines
Since the ranks of the Open Wire Testboardmen are thinning, and Open Wire Telegraph lines are almost non-existent today, it may be of interest to describe some of the testing techniques employed by those who did such work.
Western Union, the railroads, AT&T and other companies had extensive open wire plants spanning the U.S. up until the 1960's. Technological advances have largely supplanted the open wire lines and over-come their vulnerability to disaster.
The old-time Testboardman or Wire Chief had an interesting job to do, especially when the weather was less than ideal and rain, sleet, snow and ice wreaked havoc on the lines. Then, too, Lightning often walked about, seeking arbitrary paths to Mother Earth, and what better place to go than a nice fat Telegraph wire strung high on wet poles. "Atmospheric" electricity was something to be reckoned with indeed, and it caused many a Wire Chief to tear his hair.
When the weather was fine over his district, the Wire Chief usually did routine tests once a day which were intended to determine the basic "health" of the wires traversing his territory. Routine measurements conducted at one end of the line, with a distant office assisting, were conducted to measure the leakage to ground or "insulation resistance" of each wire, continuity of same, and working current in the circuit assigned to the wire if any. These measurements were usually recorded at intervals perhaps daily, or weekly or even monthly depending on the particular Company's policy. These records were kept for reference, and could be referred to as a "standard" or benchmark when things went wrong.
This testing was service-interrupting, so it was normally done during the "low usage" part of the day when the wire was idle. If this was not possible, service would be maintained by "patching" the circuit carried by a given wire off to a known good spare wire while the testing was done.
The primary testing instrument on a typical telegraph switchboard was the ES7A Volt-Mil-ammeter (VMA). The Switchboard Volt-mil-ammeter was a special "zero center" Galvanometer (micro-ammeter) that was calibrated to 200 units each side of center. A special set of shunt resistances, along with a jack and plug arrangement allowed the meter to be quickly set up as either a voltmeter or a mil-ammeter, with a full scale reading of 200 volts, or 200 milliamperes either side of center, as desired. When set up as a voltmeter, special testing potentials of up to plus 200 volts, minus 200 volts, or ground, could be connected to one side of the meter, with the other side of the meter connected to a test cord with a plug that fit the line jacks on the board. A strap key was also provided that could reverse the testing battery potential applied to the meter. When set up as a mil-ammeter, both sides of the meter were connected to the test cord. Current value in milliamperes and direction could then be easily observed by plugging the test cord into a series jack in the switchboard jack circuit of any given wire.
