Frederick A. Draper.
A set of telegraph instruments adequate for good work on a short line may be easily and cheaply made, and much interesting and profitable information obtained therefrom.
As a preliminary to experiments in wireless telegraphy, the work here required would be most valuable, a thorough understanding of all the different parts of ordinary instruments being absolutely necessary to satisfactory results with the wireless. The parts here described, consisting of key, sounder and battery, are patterned closely after the instruments in regular use, having the different adjustments, from the use of which may be learned all that an expensive outfit would supply. A fret-saw would be very useful in the work, but may be dispensed with.
The key and sounder are made upon a base, A, of any suitable wood, 8" long, 5" wide and 1/2" thick. The key will be first described. The two supports, B, are 3/8" thick, 1 3/4" long, 5/8" wide, cut in to form shoulders at the lower end. These are fitted to holes cut in the base A, 1 1/8" apart, the outer one being 1 1/2" from the right end. The lever C, at the part between the supports, is 1 1/4" wide and 5" long; preferably of the shape shown in Fig. 1, but may be a straight piece. A round-headed brass screw, F, is put through, 23/8" from the front end, the wire M being carried around the screw under the head. The brass screw-eye G is
§" from the other end of the lever, and serves to adjust the degree of movement desired for the lever. A wire nail, J, holds the lever in place. It should fit tightly into the lever, but have play enough in the holes in the supports to move without friction. A small hole, O, is bored through the lever, for the wire M to be run through loosely to the under side of the lever to the terminal screw H. The terminal screw E is put through a section of a common thread-spool or other round fig. 2 section of wood, which has been glued to the base. The space is left for the arm D to slip under the head of the screw and make a good contact. The top of the head is filed off a little, to make a flat contact with the point of the screw F, which has also been filed flat. A 1" hole, P, is bored in the under side of the base, to allow the screw E to project far enough to attach the wire connection. Two or three turns of bare wire around the point of the screw will answer, but soldering would be better. The compression spring L keeps the lever up, except when pressure is applied in transmitting messages. A rubber band attached to the other end may be used, if this spring is not easily procured. The spring is kept in place by the ends, which are straightened out to fit into holes in the lever C and base A. These holes can be made with a small awl.
The circuit-closer D consists of a strip of brass, held in place by the brass screw H, which passes through one end of a piece of wood, 1 3/8" long, 5/8" wide and 1/4" thick, into the base A and the hole P, allowing the end of the screw to be used to attach connecting wire. The other end of the wooden block serves as a rest for the point of the screw-eye G, a flat-headed brass screw being put through to hold the wood in place and prevent excessive wear from the screw-eye. The other end of D is bent a quarter turn, to form a resting-place for the finger when opening or closing the circuit. The wire M is 16 or 18 double-covered copper wire. The ends being stripped of the covering, one end is carried around the contact screw F, and the other end through a small hole in the base, to the point of the terminal screw H, the screw F being turned tight to hold the wire in place. The connections of the screw H and wire should be soldered, if possible. Four brass-head upholstering nails, one under each corner of the base, make good legs, and prevent the connecting wires from being injured. Regulate the screw-eye G, so that the lever C has sufficient play to separate the point of screw F, about 1 1/6" from screw E.
The only part of the sounder requiring special care in construction is the electro-magnet D. This should be made very carefully, as upon its proper working depends the success of the whole apparatus. The function of the electro-magnet, when excited by an electric current, is to attract the armature F, this movement of the armature making the "click," which the experienced operator recognizes, and so reads the message that is being transmitted. The well-known "horseshoe," or permanent magnet, attracts pieces of iron, and holds them in close contact until removed. An electro-magnet differs, inasmuch as it only has
Fig. 3 attractive power while excited by the electric current. If an electro-magnet does not at once lose power when the current ceases, it is evident that the iron core is not of soft enough iron.
An electro-magnet, as here described, consists of the iron cores, G, the connecting iron base, H, the wiring, D, and nonconducting face-plates, E. The cores must be of very soft iron, 13/4|" long and 1/4" diameter. Iron rivets will answer, though Norway bar iron would be better. If any doubt exists as to the iron being soft enough for the purpose, by placing it in a coal fire in the stove and heating it to a red heat, and then letting it get cold as the fire dies out, the required softness will be obtained. Hard iron will not be suitable, as it retains magnetism imparted to it by the electric current, while soft iron does not. The core must quickly demagnetize, otherwise the armature would not immediately separate from the core after being attracted to it, making it impossible to correctly transmit the signals. Having obtained suitable cores, they should have the lower ends filed down to fit holes drilled in the base H, which consists of a flat piece of soft iron, 2" long, 1/2" wide and 1/4" thick. The holes should be 13/8|" between centers, and 3/16" in diameter. An additional hole is bored in the center of H to receive the screw S. The cores can be filed down to fit, with the ends projecting slightly through the holes, and after being wound with wire, permanently held in place by carefully hammering a flange or head. After fitting the cores, they are removed and fitted with the insulating material. This consists of a round ebonite washer at each end, 1" in diameter, with the center hole, through which the core passes, made a snug fit. Two small holes are drilled in the washer at the lower end of the core, one hole close to the core hole, and the other 1/8" from the outside edge. These holes are for the ends of the coil wire to pass through. These washers are 11/8" apart. Between them, and around the core, are wrapped three or four layers of waxed paper, care being taken to have the edges of the paper meet the washers, or, if available, a piece of ebonite tubing may be used. They should then be wound, beginning at the lower end, with No. 22 or 24 double silk-covered wire, laid on in even layers until the diameter of the coil reaches 3/4", the end of the last layer being at the same end of the core as the first end. Wind regularly, and evenly. Any slight ridges may be corrected in the last layer by winding a strip of note paper once around the next-to-the-last layer,' and winding the last layer over the paper. Three inches of wire should be left at each end for connections, as hereafter described. One coil is wound in the opposite direction from the other, the wiring of the two coils when upright taking the direction of the letter . Another way of determining the direction of the wiring is by putting the two lower ends of the core together, to form a straight line. The wiring on both cores should be in the same direction as though they were a single piece. The winding completed, give the outside layer a coating of shellac, which will prevent moisture from interfering with the working. The appearance can be improved by covering with a strip of leath-eret, the kind used by bookbinders for book covers. The coils are then fastened to the base H, the two outside ends of the wires being cut a suitable length, stripped of the covering, twisted together and soldered. (See Fig. 3.) To prevent any possibility of leakage of the current from the wire to the iron base, wrap the joint with twine, and cover with wax, or with bell-hanger's insulating tape. The electro-magnet is now complete, and may be put aside until the rest of the sounder is ready.