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    Electromagnets in Telegraph Instruments

    by L. E. “Ed” Trump, AL7N

    (Originally Published: Morsum Magnificat, June 1996)

    There are several things that need to be understood about the magnet coils in telegraph instruments, particularly if one is rewinding magnets during restoration of original instruments or building new instruments.
    The magnet spools have a soft iron core and press-fit spool ends made from hard rubber or Bakelite or other insulating material.  Some instruments have the coils covered with hard rubber or sheet brass coil covers which improve the appearance of the instrument.
    The magnet cores are connected together at the bottom by a soft iron “heel piece”, usually held to the cores by iron screws.  The heel piece also usually mounts the entire coil assembly to the rest of the instrument.
    The sounder or relay armature, also made from soft iron and attached to the pivoted sounder bar or relay tongue, serves as the final component of the “magnetic Circuit” of the instrument.  See Fig. 1 and Fig. 2.

“Magnetic Circuit”
    The “magnetic circuit” is formed when current is flowing in the coil windings so as to cause each magnet to develop and “north” and a “south” pole.  The armature is allowed to be pulled close to the magnet spool ends, but not allowed to touch, lest it become temporarily magnetized and fail to release when the current stops flowing.
    Better quality instruments have small copper pin set in the ends of the core pieces to prevent such armature “sealing” or touching regardless of instrument maladjustment.  The heel piece serves to connect the north and south poles together at the lower ends of the magnets, and leaves the moveable armature at the top to complete the magnetic circuit.

Coil Windings the Same
    The coil windings are placed on the magnet spools exactly the same for both magnets.  The way the ends of the coil wires are connected determines the “correctness” of the magnetic circuit. i.e., the proper orientation of the north and south poles of each magnet.
    We must have a north pole and a south pole at the top of each magnet core pair to produce the strongest possible magnetic circuit and ensure satisfactory operation of the instrument.  See Fig. 3.

    The way the coil leads are connected into circuit determines this, assuming that both coils are wound in the same direction.  Since winding coils involves considerable labor, winding them in the same direction makes good sense from the standpoint of consistency and relative ease of production, especially if several are to be done at the same time.

    The coil windings must be connected correctly, as shown in Fig. 4, to produce the desired results.  Improper connections will result in the instrument operating poorly or not at all.

  The magnet windings may also be connected in parallel instead of series.  This produces the same magnetic circuit, although the current in the circuit must double in order to produce the same magnetic pull on the instrument armature.
    This is because the current in the circuit divides and only half passes through each coil winding in the parallel arrangement.  See Figs. 5 and 6.

Resistance Values
    The magnet coils may be wound to any DC resistance desired.  Original instruments intended for commercial use are usually wound to a resistance of 50 to 75 Ohms and when series-connected present a total resistance of 100 to 150 Ohms to circuit.
    This value is necessary for good operation over lengthy lines that have fairly large series resistance, shunt leakage losses and operating currents in the range of 30 to 60mA.
    Inexpensive practice sets were usually made with co8ils wound with heavier wire and coil resistance of 4 to 20 Ohms total.  These sets were made to be used on short metallic loops and powered by a few cells of gravity battery or dry cells, and were designed to operate at currents of 200 to 250mA or so.
    These instruments are not compatible in circuit with commercial main line instruments because of their current requirements.  The basic engineering considerations about how the coils are wound still apply, however.  Four-ohm local sounders intended to be used with a gravity cell local battery also fall into this class of instruments.

Wire to use    
    A magnet coil wound to 75 Ohms DC resistance will use about 350 feet of No. 32 enameled magnet wire.  No. 30 wire could also be used but the magnets will be larger in size since more of the larger size wire would have to be wound for the same resistance value.
    Wire larger than No. 30 would not be practical due to the size of the resulting magnet.  Wire smaller than No. 32 is more susceptible to breakage and damage from excessive current and thus is also not a good choice for this purpose.  A single layer of paper is placed on the bare iron core before winding is begun.
    (Note: Wire sizes mentioned are American Wire Gauge.  See Table 1 for corresponding Imperial and metric diameter, and also nearest equivalent British Standard Wire Gauges. - Ed.)

Winding the Coils
    Winding the magnet coil is a straight forward if somewhat tedious operation.  The spool must be turned either by hand crank or electric motor.  A pin with a threaded stud on the end that fits the tapped hole in the lower end of the core piece is chucked into the turning device, and the opposite end of the magnet spool is supported by a fixture that allows the end of the magnet core that protrudes through the top spool-end to slip into it.
    The magnet wire is fed through the hole closest to the core on the bottom spool-end and a sufficient length (six inches or so) is wound around the turning shaft and secured with tape.  This will be the pigtail lead for the “inside” end of the coil.  The wire should be covered with a flexible insulating sleeve of some sort where it passes through the spool-end to prevent premature breakage at this point and also to insulate it where it passes through the instruments bottom plate.
    It is convenient to position the magnet wire supply spool so that the wire can be fed from it with one hand while the other hand operates the turning device.  The spool is than rotated as necessary and the magnet wire is wound as evenly as possible until the spool is full or the required length of wire has been laid upon it.

Both Coils the Same
    When the spool is wound full, the end of the magnet wire should be at the spool bottom near the exit hole close to the outer edge of the lower spool-end.  Feed the end of the wire through the hole, again applying protective flexible sleeving to prevent breakage.
    The coil winding itself can be varnished and/or covered with a paper or cloth covering as desired.  The coil is now completed.  It can be removed from the winding jig and the coil cover slipped over it if one is to be used.
    The other coil should be wound exactly the same way.  Both coils for a given instrument should be as nearly alike as possible.  When they are both finished, they can be attached to the heel piece, and the coil assembly can be mounted on the instrument.  The winding pigtail leads can then be connected as desired on the finished instrument.

Copyright L. E. “Ed” Trump, used by permission of author.



In a recent e-mail about winding coils, Ed Trump wrote the following:

I have done this several times, both for new coils and rewinds of originals.

What I did was make a wood frame winding jig with a blind hole on one end (left) upright, and a hole to pass a metal shaft on the other (right) end upright.

The metal shaft is free to slide left and right to accommodate different lengths of coil spools.  The blind hole at the left end serves to support the "top" end of the spool while rewinding.
After repairing or replacing/fabricating the coil spool ends, drilling the two small holes in the bottom one for the magnet wire to pass thru, and pressing them tightly onto the magnet core piece, I suspended the spool of rewind wire above the jig on a rod so that it can feed wire freely toward the coil being rewound.

The core piece with spool ends is screwed onto a stud in the end of the metal shaft with appropriate threads and tightened into place.

 A movable collar on the shaft is placed to hold the spool in the jig so it can turn with a little drag, and set with setscrews to hold it in place.

If you study original sounder or relay magnets, you will see that they are both wound in the SAME direction.  Doesn't matter which, just as long as they are identical in winding direction.
I fashioned a hand crank on the right end of the shaft so I can turn it with my right hand and feed the magnet wire off the supply spool with my left hand. I turn my coil windings clockwise as facing the right end of the winding jig.

I tried using a variable speed electrical drill, but it was too hard to control and tended to cause excessive breakage in the small magnet wire which is not good. Although tedious, I got the best results by turning the core by hand.  Keep some masking tape handy to hold things in place when you want to take a break.  It takes a while to wind a full magnet to about 75 Ohms worth of No. 30 wire!

A single layer of light paper should be placed on the magnet core piece before laying on the windings, The core must be insulated from the coil circuit.

Enameled magnet wire probably is easier to get today than DSC (double silk covered) wire like original instuments used, or wire salvaged from damaged magnets can be reused if it is in good condition (not burned nor broken) and carefully unwound and rewound on the supply spool.
Feed the magnet wire thru the hole in the bottom spool end nearest the core from the inside, and wind Six or eight inches around the shaft and secure it with a piece of tape.   Then start winding.
The windings should be placed in tight even layers right to left, with all turns on the same layer adjacent to one another. Start a new layer when you get to the other end of the spool and wind back towards the right.  Then continue until the spool is nearly full.

Mainline instrument magnets wound with No.30 or 32 magnet wire will be at about the right resistance when the magnet spool is nearly full.  Of course low resistance 4 Ohm local sounders will use larger wire, something like No. 22, 24 or 26.  Run the end of the winding through the ouside hole in the bottom spool end with a pigtail length of six inches or so, and you are done.

Then wind the other magnet in exactly the same way and to the same winding depth.  I place a cloth “jacket” over the completed windings, and then the coil covers if they are to be used.

After this, the magnets spools can be reassembled to the heel piece and mounted on the instrument frame. The iron heel piece or strap that ties the bottoms of the spools together is an essential part of the magnet assembly and serves to complete the “magnetic circuit” of the instrument so it will operate correctly.

Now, the electrical connections to the magnet windings are very important. The “inside” coil pigtails (the ones nearest the core) should be connected together and then the connections made to the instrument binding posts with the “outside” pigtail ends.

This ensures that the “magnetic circuit” will be proper to act on the instrument armature and will cause one top spool end to be a “north” pole, and the top end of the other spool to be a “south” pole when current is flowing through the coils, which is what you want.
Good luck,  I hope this was helpful.
Ed Trump
FB Fairbanks

Copyright 2013.  Used by permission of author

Webmaster note: Cloth covered wire may be obtained at Antique Electronic Supply <>