This article came from ArtiFax Books in 2001.
Found at the Internet Archive.
Enjoy
Using vintage telegraph instruments:
Applying power to old sounders.
Tom French
W1IMQ
"the McElroy collector"
Always interested in acquiring T.R. McElroy instruments, catalogs and paper.
Contact me if you have any for sale or trade.August 2001
Collecting old landline telegraph keys, sounders and relays is an
interesting hobby. These historic communication instruments were "state
of the art" back in the 1800s, and were still in use right up through
the 1950s and later. They bring visions of the American West and
railroads. They make interesting display pieces.
More interesting is to hook them up to a battery and try them
out. Re-create the sound of the Western Union office and the old
railroad depot. On this page I will try to help you check out and power
up your sounders. References to back issues of The Vail Correspondent
(TVC) are given for those who want to research this field in more
detail.
The typical sounder is simply two electromagnets (coils of wire on an
iron core) set up to attract a steel
bar. This bar is attached to a
non-magnetic (brass or aluminum) lever. When the magnets are activated
the lever is pulled down, producing a "click" when the lever is stopped
by an adjustable screw. When the circuit is broken, a spring returns the
lever to its original position, causing a "clack" as the lever hits
another stop screw.These sounds, and the time interval between them, are
used to send a message in Morse Code. (Reference: TVC8.)
A sounder should be adjusted so that a small gap (the thickness
of a sheet of paper is enough) is left between the bar and the magnets
when the sounder is energized. Then adjust the upper lever stop screw so
there is about 1/16 inch of lever travel. Finally, adjust the spring so
that the lever moves easily. (The last two adjustments can be refined
later to produce the most pleasing sound.)
Before you can get a sounder to work there must be electrical
continuity in the wiring, from one wire terminal to the other. A quick
test with a 3 to 6 volt battery may do -- but just touch the battery
wires to the terminals quickly, since this may be too much voltage for
some sounders. On the other hand, it may not be enough voltage for other
sounders; if the lever does not pull in, press it down and see if it's
held by the magnets. If so, the circuit is complete.
Or you can test the wiring with an ohmmeter. This will also tell
you (if the wiring is good) the resistance of the sounder, which you
will need to know. If the battery or meter test is not successful, it
may be simply that the corrosion on the wires and terminals will not
permit the proper result. In that case, clean the surfaces. Broken wires
are common, and will have to be repaired.
Now determine the sounder resistance. This is usually stamped on
the top or side of the instrument. If not, use your ohmmeter. Sounders
are current-operated instruments. Generally the following applies (these
are not critical; a 25% variation would not be harmful to the
instrument):
... 4-ohm instruments are intended for 250 milliampere lines, and require 1 volt;
... 20 ohm instruments require 175 ma (3.5 volts); and
... 150 ohm main line instruments require 40 ma (6 volts).
In the old days, gravity batteries ("crowfoot battery") might be used.
One gravity cell puts out about 1.08 volts. A 4-ohm sounder in my
collection has a Western Union label on it that says "This sounder is
designed for use with one cell only of gravity battery." (Batteries are
discussed in TVC17). For home use, common dry cells are sufficient. You
might, for example, use one or two D cells (1.5 to 3 volts) for a 4-ohm
sounder. Putting a milliammeter in the circuit will be helpful. Of
course, a vintage Western Union meter is best!
You may find your sounder to be "sluggish" even though you have
applied the proper voltage; this is more often the case with
high-resistance (main line) instruments -- 100 to 150 ohms. You may be
tempted to increase the voltage to get a good sound out of it. But this
is the wrong approach. The sluggishness of operation is due to the
inductance of the magnet coils and the relatively long time constant of
the circuit. The time constant is proportional to the inductance, and
inversely proportional to the resistance of the circuit: T=L/R.
To get "snappy" sounder operation, you should decrease the time
constant. Looking at the formula, this is done by increasing the
resistance of the circuit (adding a series resistor, or so-called
"ballast"). Now, in order to get the required current again, you may
increase the voltage (see TVC10). Use non-inductive resistors, of
course.
I hope this information will help you get more enjoyment out of
your collecting. More information on Morse instruments appears in the
"Landline" column of TVC. (Note, TVC is no longer published, although
back issues are available. See the TVC page on this web site.) Landline
telegraph instruments, circuits and practices are also included in the
"Railroad Telegrapher's Handbook" and in Prescott's "History, Theory and
Practice of the Electric Telegraph".