Home-made Electrical Apparatus

by Alfred Powell Morgan

CHAPTER VIII. HOW TO MAKE A TELEGRAPH KEY AND SOUNDER AND INSTALL A TELEGRAPH LINE.

The modern telegraph consists essentially of four things:

1. A battery which produces the electric current.
2. A wire to conduct the current from one point to another.
3. A key or switch for turning the current off and on.
4. An electro-magnetic receiving apparatus, which changes the
pulsations of the electric current into sounds.

[Illustration: FIG. 83.—Key Frame.]

Its operation is not, as many suppose, complicated and difficult to understand, but is quite simple.

The key is really a form of switch and is simply a contrivance for controlling the electric current. It consists of a steel lever, pivoted in the centre, and provided with a rubber knob, which the operator grasps lightly with the thumb and forefinger. When the lever is pressed downwards, a platinum point on the under side is brought into contact with another similar point set into a rubber bushing in the base of the key, so that there is no electrical connection between the two points unless the key is pressed down or "closed" as it is usually termed.

The receiving device, or "sounder," consists of two electro-magnets mounted on a base under a flat strip or iron, called the armature, which is attracted by the magnetism of the electro-magnets when a current flows through them, and is withdrawn by a spring when no magnetism is produced by the windings.

[Illustration: FIG. 84.—Sounder Frame.]

The armature is usually mounted on a strip of brass or aluminum called the "lever". The lever strikes against an anvil and produces the clicks which form the dots and dashes of the telegraph alphabet.

Every time the key is pressed, an electric current goes through the line, causing the magnets to draw the armature downward and produce a click. When the key lever is released, the current is shut off and the lever flies up and clicks against the top of the anvil.

The time between the first and second clicks may be easily varied at will, by changing the length of time that the key lever is held down. Two clicks very close together, made by pressing the key and immediately releasing it, are called a "dot". A "dash" is made by pressing the key down and holding it down for a fraction of a second so that the interval between the first and second clicks is about three times as long as that between the clicks of a dot.

[Illustration: FIG. 85.—The Electro Magnets.]

Any boy can make a simple telegraph set quite easily. The only drawback of such an apparatus, however, is that it is usually not practical for long lines but may only be used for ticking messages from one room to another.

It is equally possible for the young experimenter to make a perfectly practical telegraph outfit which will operate over a line a mile long, if he is willing to give the time and labor necessary to the work.

The telegraph set-described below is one which, because of its simplicity, particularly lends itself to construction by the young experimenter whose tools are rather limited.

In order to make a really practical telegraph outfit, it is almost essential that the key and sounder frame be made out of cast iron so as to give the instruments the requisite stiffness. It is easier to use castings than to attempt to bend the parts out of metal.

In order to secure the proper castings it will be necessary to first make a set of "patterns". This is not very difficult.

The key frame is shown in Figure 83 and the Sounder frame in Figure 84. The patterns should be cut out of wood according to the exact shape and dimensions shown in the illustrations. Omit all the holes, however, because no holes should appear in the patterns but only in the finished castings.

[Illustration: FIG. 86—The Sounder Armature.]

The patterns should be made perfectly smooth and be a good, clean job throughout. After they are finished, give them each a coat of shellac and they are ready for the foundry. Almost any iron foundry will make up the castings if supplied with the patterns. If a number of experimenters "chip in" and make up a set of patterns from which several sets of castings are ordered at the same time, the cost will be very slight.

After the castings are secured, they should be cleaned up with a file and drilled according to the illustration in Figures 83 and 84. Several of the holes should be tapped to receive an 8-30 machine screw. Such holes are marked in the illustrations.

The holes not marked "Tap" should be just large enough to allow an 8-32 screw to slip through.

[Illustration: FIG. 87.—Sounder Frame with Lever in Position.]

*The Electro-magnets* are illustrated in Figure 85. The magnet core is made of a piece of 5-16 inch round iron rod an inch and a quarter long. A fibre washer, three-quarters of an inch in diameter is fitted over each end of the core. One end of the core is drilled and tapped to receive an 8-32 screw.

The magnet bobbin, before winding is shown at the left of the illustration and a finished magnet at the right. Two magnets are required. They should both be wound in the same direction with No. 25 single cotton covered wire. The wire should be wound in smooth, even layers and the terminals led out through two small holes in the fibre heads.

*The Sounder Armature* is a piece of steel or iron rod, five-sixteenths of an inch square and three and three-eighths inches long. It serves both as the armature and the sounder lever. The exact location of the holes is shown in Figure 86. The holes "h" and "i" should be tapped to receive an 8-32 screw. The hole "j" should pass a screw of the same size. This screw serves as the pivot for the armature. Figure 87 shows how the sounder frame should appear when the lever is in position. The screw A controls the distance which the armature moves on its upward stroke. The screw B strikes the "anvil" on the downward stroke and controls the distance between the armature and the electro-magnets.

[Illustration: FIG. 88.—Top View of Completed Instrument]

The armature is pushed upwards and held away from the magnets against the screw A by a small spiral spring. The lower end of the spring sets in the hole "C" in the sounder frame and the upper end rests against the end of the screw C so that the tension of the spring can be regulated by moving the screw up and down. The armature should be free to move up and down on the pivot screw but should not move sideways.

*The Base* of the instrument is a rectangular piece of hardwood, four and one-half inches long, three and three-quarter inches wide and one-half inch thick. The sounder is mounted on the base in the position shown in Figure 88, which is a top view of the complete instrument. Two long 8-32 machine screws pass upwards through the wooden base, through the holes "e" and "f" in the sounder frame and into the bottom of the magnets, thus serving not only to hold the magnets in position but also the sounder frame to the base.

[Illustration: FIG. 89.—Side View of Key.]

Figure 89 is a side view of the completed key. The key lever and the circuit closing lever are illustrated in Figure 90. The lever is four and three-eighths inches long and five-sixteenths of an inch wide. It should be made from spring brass. The circuit closing lever is a strip of brass of the same width. All the holes in both levers, with the exception of the centre hole in the key lever should be large enough to pass an 8-32 screw. The centre hole should be tapped to receive an 8-32 screw.

The key lever is fastened to the frame by means of a round headed 8-32 machine screw. The screw should be tightened so that the lever cannot move from side to side. The circuit closing lever is pivoted to the back end so that it can swing. The key lever should be fitted with a regular key knob and the circuit closing lever with a small knob such as is generally used for the purpose. Both of these styles of knobs can be purchased from almost any electrical supply house.

The contact points are best made of silver. Silver makes a better contact metal than almost anything else, with the exception of platinum. If silver cannot be obtained a piece of large german-silver wire may be used.

[Illustration: FIG. 90.—Key and Circuit Closing Levers.]

One contact is soldered to the under side of the key lever. The other contact should be mounted on the head of a screw and pass through a hole in the iron key frame and through the wooden base underneath. This lower contact should be directly underneath the upper contact and be thoroughly insulated from the base. Several thicknesses of well shellaced paper will form a first class insulating medium. The length of the contacts should be such that there is a distance of about 3-32 of an inch between them when the lever is horizontal.

The circuit closing lever should make contact with a small strip of spring brass under the lower contact point so that the contact is connected to the frame of the key through the circuit closing lever when the latter is "closed".

The adjustment screw on the key frame is used to regulate the upward stroke of the key lever.

The adjustment screw on the key lever is used to regulate the spring tension of the lever. The lower end rests against the top of a small spiral spring made of brass wire.

[Illustration: FIG. 91.—American Morse Code.]

Connecting the outfit is a very simple mater. The two inside terminals of the sounder magnets should be connected together. One of the outside terminals is led to a binding post mounted on the back of the base. The other terminal is connected to the lower key contact. The frame of the key is connected to a second binding post at the back of the base. The connecting wires should pass through small holes and along the under side of the base.

The Morse Telegraph Code is shown in Figure 91.

A single dry cell will provide sufficient current for practising the Code. The cell should be connected to the two binding posts and the circuit closing lever should be kept open at all times or the battery will become exhausted.

[Illustration: FIG. 92.—Circuit for Two Instruments.]

The number of batteries required on a long line will depend upon the resistance of the line and cannot easily be pre-determined.

Figure 92 shows a circuit for connecting two instruments on a line. When the double contact switch is on point A the batteries are out of circuit. Throwing the switch on B puts the batteries into circuit. They should be kept out of circuit when the stations are not in use. The circuit closer at Station 1 must be kept closed when Station 2 is telegraphing and vice versa.