What in blue blazes is a polar duplex half-repeater anyway, arranged for dial-up or anything else? That's a perfectly reasonable question; not everyone is privy to the mysteries hidden within the doors of the telegraph repeater or T&R departments. For an answer, let's take a look at what was the golden age of telegraphy, full of the excitement of discovery and innovation. In the latter part of the l9th century, a matter of considerable interest to the high and mighty of the telegraph business was squeezing more traffic through, (hence greater returns on investment in) the existing wire plant, which was by far the most capital-intensive part of the system. The leading "electricians" of the day devoted considerable thought to this problem and from their efforts many elegant and ingenious designs emerged.
As Tom Lockwood writes in his Practical Information for Telephonists, 1882, "The first repeater was the button invented and applied at Auburn, N.Y. Since then Bulkley, Farmer & Woodman, Hicks, Milliken, Bunnell, Gatlin and many others have invented repeaters: and indeed, up to the time when Stearns perfected the duplex, every young operator as soon as he could rush thirty words per minute, conceived it his duty forthwith to invent a repeater. Since that era, the duplex has succeeded it in that department of innovation."
One of the more successful and enduring of these developments was the Bridge Polar Duplex. The duplex, of course, permitted simultaneous transmission in both directions. This led to the situation, so we're told, that required the operator working the sending side to do the receiver's breaking for him or her, thus yielding razzberries to embarassment in a ratio of three to one.
The bridge polar duplex made use of Wheatstone's balanced bridge to permit signalling in both directions combined with reversals of the line current for signal transmission. This polar, or double-current operation as it was called, proved much superior to the single current, open-and-close working of the ordinary Morse wire.
Instead of waiting around for the wire to assume its steady-state condition, putting reverse battery to the line gave it a substantial shove. Since the receiving relays depended on direction of the current through them, rather than magnitude, a system resulted that was very reliable under a wide range of adverse line conditions.
It was soon observed that even after conditions had deteriorated to the point where the bridge polar would no longer work duplex, it still worked quite well in one direction or the other. From this it was only a short step to rearrange things so that the duplex sets could be made to work single. This was done by the application of a set of instruments which constituted half of a single line repeater, the other half being the distant end. Hence, half-repeater, or half-set. It is easy to see the derivation of the term "half-duplex" still in use today.
The bridge polar duplex was adopted by AT&T as its standard in the early years of the present century and served as the workhorse of its intercity telegraph network for decades, gradually being phased out in favor of full metallic and voice frequency carrier systems. Athearn-type single line repeaters formed a compliment for interconnection and short haul work. Some of these sets were still around as late as the 1940s.
The photo above was taken on the seventh floor of Philadelphia's Bourse Building in 1920, it shows over 100 duplex sets. The duplex apparatus was mounted on long mahogany tables with the Athearns on shelves above. These sets were so arranged that any kind of line facility or circuit configuration would be accommodated, hence the numerous table switches in evidence.
From the raised platform on the left, called the "pulpit", telegraph repeater attendants kept watch for calling-in signal lamps at the repeaters, brought in by subscribers experiencing trouble. The TRA would, on spying a lamp, work his way over to the set and do what was necessary to get the customer back in business. It was not unknown for certain of the attendants, not quite up to the job, to saunter over, kill the lamp and slip away, leaving it for some other poor fish to catch double grief. Or so it is related.
One can well understand the usefulness of a wooden "listening stick" in picking out the sound of one particular relay amidst all that clatter. The small end is placed against the desired instrument, which can then be heard distinctly by holding the round "receiver", a hollow chamber fastened to the stick at a 45 degree angle, to the ear.
Finally, we get to the dial-up net. While the tables have long since disappeared into basement workshops and the line balancing apparatus gone to the junkman, many of the component instruments survive. Perhaps they're not fully understood or appreciated by those who have them. The photo shows them in their "natural" arrangement. Since the repeater is fixed in the half-duplex mode, there is no need for table switches, nor is there any need for bridge coils and artificial lines. A modem and the power supplies are separate units connected by short cables.
The W.E. 3OA polar relay on the right side of the set follows signals on the modem's Receive Data lead. The relay's 400 ohm winding is in series with a 30-0-30 milliampere meter. While the relay is sensitive enough to respond directly to the modem, the current would be too low to give the meter much play. For that reason a solid state polar "amplifier" is incorporated, to give a swing of about 25 ma. each side. Strictly a case of using a lot of the steam to blow the whistle.
The contacts of the polar relay drive a "control" relay (W.E. 24A) or "silent transmitter" as they are sometimes called. This relay serves to repeat incoming signals to the local loop to the subscriber, while holding the polechanger (W.E. 25A) to its left closed on space.
Directly in front of the 24A is a 140 ohm W.E. 30 sounder. Its windings are in series with those of the control relay and it acts to monitor the received signals. The pole-changing relay (25-A) responds to signals sent in the local loop. It is identical to one-half of an Athearn repeater, with the exception of having contacts on the back side of the tongue as well as the front, and applies outgoing polar signals to the modem's Transmit Data lead. This is analogous to sending into the bridge apex.
At the left front is a back-contact repeating sounder (W.E. 4A)that follows the operation of the polechanger. It has two functions; that of a monitoring sounder for outgoing signals, and more importantly, that of insuring a clean break toward the distant end when fast signals are being received by shunting the polar relay contacts, thus locking up the control relay so that the pole changer can make its full excursion to spacing battery.
It is the precursor of the much-maligned break relay of later years. "Changed break relay and OK. Put regular", regardless of the the true nature of the trouble. Much embarassment was spared in this way.
Operation at the subscriber's set is the same as on any single Morse circuit. At the repeater, however, incoming and outgoing signals take different paths, so there is a distinctive difference in sound as the directions of transmission changes.
It is still possible, then, for us to enjoy this echo from the past, if only on a modest scale. To experience the sound of a room full of these instruments at a busy hour must, unfortunately, be left to the imagination. These same results can be obtained with other combinations of instruments, of course. 73.
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