R. C. Browne.

For detecting the messages that are sent out by the apparatus described in the last article, we will have to build a set of receiving instruments consisting of the well-known telegraph sounder, and relay of 250 or more ohms resistance. The higher the resistance of the relay the more sensitive the apparatus. In addition to these we must make what is known as a coherer and decoherer. The coherer is simple in construction, and based upon the property the electrical waves have of causing loose particles of iron filings to attach themselves together, somewhat similar to the action of iron filings when magnetized.

From a glass tube of about 1/8" bore and medium thick walls, cut a piece 2" long. This can easily be done by scratching the tube at the proper place with a file, and then applying pressure on each side of the scratch by holding it firmly in the hands with the two thumbs just opposite the scratch, and boldly pressing the tube over them.

From a piece of round brass rod cut or turn two little plugs about 3/8" long and of a diameter to snugly fit the hole in the glass tube. Brass rods can easily be cut with a hack-saw. To one end of each plug solder the eye of a long, slim needle, first breaking off or blunting the points, to avoid pricking the fingers.

A small hole can be drilled in one end of each plug, to facilitate soldering of the needle. Obtain two double binding-posts and mount them on a baseboard 7" long, 5" wide and %" thick. They should be placed 1" from one side of the board, 21/2" apart and 21/2" from one end, as shown in

Fig. 1. Have the holes facing each other, so that the needles passing through them will enter the glass tube and hold it firmly. Before putting the plugs into the tube make a small quantity of rather coarse iron filings by forcibly filing a piece of clean, dry iron with a coarse cross-cut file which is perfectly clean and dry. These filings are to be placed in the glass tube between the two brass plugs, care being taken not to get too large an amount, and to avoid handling them with the fingers or squeezing them between the brass plugs. The best way to get the filings into the tube is to make them over a clean piece of paper with a fold across it, so the filings can easily be poured into the tube. The plugs should be from 1/16" to 1/8" apart, and the space between them not over half full of filings. A rubber band on the outside of the tube, where the hammer of the bell rests, will prevent breakage from too heavy a stroke of the bell.

Wireless Telegraphy Apparatus II Receiving Instrum 223

Fig. 3.

The decoherer, as its name implies, is an instrument to counteract the work done by the coherer; that is, it destroys the coherence or clinging together of the filings which follow the passage of the wave impulses. This is accomplished by shaking the filings in the tube; and an excellent de-coherer is obtained by mounting an electric bell (any box bells which work well will answer) on the baseboard so that the hammer of the bell, when at rest, lays against the glass tube of the coherer, as shown in Fig. 3.

I should advise the student or amateur to purchase the relay of some reliable firm. It should be of at least 250 ohms resistance ; and 500 or 1,000 will work better and receive at greater distance. But as a relay of such high resistance is rather expensive, the reader may obtain a cheap instrument of low resistance, and rewind it with finer wire. The size and amount of wire necessary for rewinding can easily be estimated by weighing the wire taken off of the coil, and looking up in a wire resistance table the size of wire having a resistance of 500 ohms or more for the same weight of wire. For most purposes the No. 36 gauge wire used in winding the induction coil described in the last paper will be found to work well.

Wireless Telegraphy Apparatus II Receiving Instrum 224

Fig. 4.

The sounder may also be purchased, but a simple one may be made as follows : Two upright brass posts 1/4' diameter (see Fig. 4), A being 23/4" long and B 21/4' long, are driven into the base board 23/16" apart. Post A has a notch 1/4" long filed 1/8" from the top; B is filed wedge-shaped, as shown.

Post C is of soft iron \" in diameter and 21/4" long, and is driven into the base exactly in the center, between the other two posts. The wooden base is 4" x 5" and 3/4" thick, 1/4" holes being bored in it for the posts. A selenoid, which is simply a paper or wooden spool wound with ten or twelve layers of No. 26 cotton-covered magnet wire, should be slipped over the iron center post. An iron lever, D, 4/' thick, 3/8" wide and 23/4" long, having a notch filed 3/8" from one end, should be placed over the coil, so that when the wedge-shaped post supports the notch in the lever, the other end of the lever should be free to move in the slot in post A. A small brass spring, made by winding thin brass wire around a wire nail, should be fastened to the base and to a hole in the short end of the lever, so as to hold the latter up against the top of the notch in A.

Wireless Telegraphy Apparatus II Receiving Instrum 225

Fig. 5.

The iron post of the coil should be driven into the base until the top is a trifle lower than the notch in the post A. The top of post B should be fa" higher than the iron post. A more elaborate sounder is described in the November, 1901, number of Amateur Work, the construction being plainly indicated by Figs. 5 and 6.

The instruments are connected as shown in Fig. 7. The coherer should be put in series with the relay and a battery of one cell. The decoherer and sounder are to be placed in multiple and then in series, with the local side of the relay and a battery of six or eight cells. One side of the coherer must be connected to the earth and the other side to an elevated wire.

For those not acquainted with the terms series and multiple, the following directions are given for making the various connections. One side of a cell of battery should be connected by wire a to one of the posts of the coherer. The other post of the coherer should be connected by b to one end of the coil of the relay; the other end of the relay coil should be connected by c to the other pole of the battery. One post on the decoherer bell should be connected by wire m to one post on the local side of relay; the other post on local side of relay connect by wire n to one side of a battery of six or eight cells connected in series; i.e., the zinc on one battery connected with the carbon of another. The other side of the battery is connected by o to the other post on decoherer (bell). The sounder should be connected to the two posts on the decoherer by wires f and p or to the wires m and o, as may be most convenient. The elevated or receiving wire should be as straight as possible. Galvanized iron telephone wire will answer. A pole or kite may be used to secure the necessary elevation; the higher the terminal end the greater the distance over which signals may be transmitted. Elevated places with sharp slopes are preferable for land operations. Over water height is desii-able, but not so necessary. The ground wire should connect to an adjacent water-pipe, though it is better to have a direct wire connection with a copper plate buried deep enough to be in earth, which is always damp. If the instruments are to remain in connection for any length of time during the season of thunder-storms, a lightning arrester should be connected between the elevated wire and the ground wire at the point where they both enter the building, the wiring being arranged to allow this. The ground wire in this case should be as large as No. 6 gauge galvanized iron.

Wireless Telegraphy Apparatus II Receiving Instrum 226

Fig. 6.

Choking coils have not been included in the apparatus here described, but they may be added if desired. According to some experimenters, they increase the sensitiveness of the instruments. They are made by filling a small glass or rubber tube say 1" long and 1/8" inside diameter, full of pieces of very fine iron wire, and then winding on the outside of the tube two layers of No. 36 cot-ton-covered magnet wire, coating the layers with shellac to hold them in place. The ends of the outside wire are connected in series with the wire connecting the coherer and relay.

When all the apparatus is completed and the various connections made, a trial may be made to see if it will work over a short distance. The coherer, sounder and relay should "click" when a common electric bell is rung within two or three feet of them. Some adjusting will undoubtedly be necessary before satisfactory results are obtained. This is true of the most expensive and delicate outfits, so we should not expect this apparatus to be an exception. Tap the coherer, to see that the filings are not packed. The relay must be adjusted very delicately, the armature as close to the poles as possible, yet not touching them. The spring should be adjusted so the tension will be sufficient, but not too strong. If the bell continues to ring after a signal, either the

Wireless Telegraphy Apparatus II Receiving Instrum 227

Fig. 7.