[Ill.u.s.tration: Fig. 7.--Two Wire Aerial.]

Connecting in the Ground.--If there is a gas or water system or a steam-heating plant in your house you can make your ground connection by clamping a ground clamp to the nearest pipe as has been previously described. Connect a length of bare or insulated copper wire with it and bring this up to the table on which you have your receiving set.

If there are no grounded pipes available then you will have to make a good ground which we shall describe presently and lead the ground wire from your receiving set out of the window and down to it.

How to Put Up a Good Aerial.--While you can use the cheap aerial already described for a small spark-coil sending set you should have a better insulated one for a 1/2 or a 1 kilowatt transformer set. The cost for the materials for a good aerial is small and when properly made and well insulated it will give results that are all out of proportion to the cost of it.

An Inexpensive Good Aerial.--A far better aerial, because it is more highly insulated, can be made by using _midget insulators_ instead of the porcelain insulators described under the caption of _A Single Wire Aerial_ and using a small _electrose leading-in insulator_ instead of the porcelain bushing. This makes a good sending aerial for small sets as well as a good receiving aerial.

The Best Aerial that Can Be Made.--To make this aerial get the following material together: (1) enough _stranded or braided wire_ for three or four lengths of parallel wires, according to the number you want to use (2) six or eight _electrose ball insulators_, see B, Fig.

8; (3) two 5-inch or 10-inch _electrose strain insulators_, see C; (4) six or eight _S-hooks_, see D; one large _withe_ with one eye for middle of end spreader, see E; (6) two smaller _withes_ with one eye each for end spreader, see E; (7) two still smaller _withes_, with two eyes each for the ends of the end spreaders, see E (8) two _thimbles_, see F, for 1/4-inch wire cable; (9) six or eight _hard rubber tubes_ or _bushings_ as shown at G; and (10) two _end spreaders_, see H; one _middle spreader_, see I; and one _leading-in spreader_, see J.

[Ill.u.s.tration: (A) Fig. 8--Part of a Good Aerial.]

[Ill.u.s.tration: (B) Fig. 8.--The Spreaders.]

For this aerial any one of a number of kinds of wire can be used and among these are (a) _stranded copper wire;_ (b) _braided copper wire;_ (c) _stranded silicon bronze wire,_ and (d) _stranded phosphor bronze wire_. Stranded and braided copper wire is very flexible as it is formed of seven strands of fine wire twisted or braided together and it is very good for short and light aerials. Silicon bronze wire is stronger than copper wire and should be used where aerials are more than 100 feet long, while phosphor bronze wire is the strongest aerial wire made and is used for high grade aerials by the commercial companies and the Government for their high-power stations.

The spreaders should be made of spruce, and should be 4 feet 10 inches long for a three-wire aerial and 7 feet 1 inch long for a four-wire aerial as the distance between the wires should be about 27 inches.

The end spreaders can be turned cylindrically but it makes a better looking job if they taper from the middle to the ends. They should be 2-1/4 inches in diameter at the middle and 1-3/4 inches at the ends.

The middle spreader can be cylindrical and 2 inches in diameter. It must have holes bored through it at equidistant points for the hard rubber tubes; each of these should be 5/8 inch in diameter and have a hole 5/32 inch in diameter through it for the aerial wire. The leading-in spreader is also made of spruce and is 1-1/2 inches square and 26 inches long. Bore three or four 5/8-inch holes at equidistant points through this spreader and insert hard rubber tubes in them as with the middle spreader.

a.s.sembling the Aerial.--Begin by measuring off the length of each wire to be used and see to it that all of them are of exactly the same length. Now push the hard rubber insulators through the holes in the middle spreader and thread the wires through the holes in the insulators as shown at A in Fig 9.

Next twist the ends of each wire to the rings of the ball insulators and then put the large withes on the middle of each of the end spreaders; fix the other withes on the spreaders so that they will be 27 inches apart and fasten the ball insulators to the eyes in the withes with the S-hooks. Now slip a thimble through the eye of one of the long strain insulators, thread a length of stranded steel wire 1/4 inch in diameter through it and fasten the ends of it to the eyes in the withes on the ends of the spreaders.

[Ill.u.s.tration: (A) Fig. 9.--Middle Spreader.]

[Ill.u.s.tration: (B) Fig. 9.--One End of Aerial Complete.]

[Ill.u.s.tration: (C) Fig. 9.--Leading in Spreader.]

Finally fasten a 40-inch length of steel stranded wire to each of the eyes of the withes on the middle of each of the spreaders, loop the other end over the thimble and then wrap the end around the wires that are fixed to the ends of the spreaders. One end of the aerial is shown complete at B in Fig. 9, and from this you can see exactly how it is a.s.sembled. Now cut off three or four pieces of wire 15 or 20 feet long and twist and solder each one to one of the aerial wires; then slip them through the hard rubber tubes in the leading-in spreader, bring their free ends together as at C and twist and solder them to a length of wire long enough to reach to your lightning switch or instruments.

Making a Good Ground.--Where you have to make a _ground_ you can do so either by (1) burying sheets of zinc or copper in the moist earth; (2) burying a number of wires in the moist earth, or (3) using a _counterpoise_. To make a ground of the first kind take half a dozen large sheets of copper or zinc, cut them into strips a foot wide, solder them all together with other strips and bury them deeply in the ground.

It is easier to make a wire ground, say of as many or more wires as you have in your aerial and connect them together with cross wires. To put such a ground in the earth you will have to use a plow to make the furrows deep enough to insure them always being moist. In the counterpoise ground you make up a system of wires exactly like your aerial, that is, you insulate them just as carefully; then you support them so that they will be as close to the ground as possible and yet not touch it or anything else. This and the other two grounds just described should be placed directly under the aerial wire if the best results are to be had. In using a counterpoise you must bring the wire from it up to and through another leading-in insulator to your instruments.

CHAPTER III

SIMPLE TELEGRAPH AND TELEPHONE RECEIVING SETS

With a crystal detector receiving set you can receive either telegraphic dots and dashes or telephonic speech and music. You can buy a receiving set already a.s.sembled or you can buy the different parts and a.s.semble them yourself. An a.s.sembled set is less bother in the beginning but if you like to experiment you can _hook up_, that is, connect the separate parts together yourself and it is perhaps a little cheaper to do it this way. Then again, by so doing you get a lot of valuable experience in wireless work and an understanding of the workings of wireless that you cannot get in any other way.

a.s.sembled Wireless Receiving Sets.--The cheapest a.s.sembled receiving set [Footnote: The Marvel, made by the Radio Mfg. Co., New York City.]

advertised is one in which the detector and tuning coil is mounted in a box. It costs $15.00, and can be bought of dealers in electric supplies generally.

This price also includes a crystal detector, an adjustable tuning coil, a single telephone receiver with head-band and the wire, porcelain insulators, lightning switch and ground clamp for the aerial wire system. It will receive wireless telegraph and telephone messages over a range of from 10 to 25 miles.

Another cheap unit receptor, that is, a complete wireless receiving set already mounted which can be used with a single aerial is sold for $25.00. [Footnote: The Aeriola Jr., made by the Westinghouse Company, Pittsburgh, Pa.] This set includes a crystal detector, a variable tuning coil, a fixed condenser and a pair of head telephone receivers.

It can also be used to receive either telegraph or telephone messages from distances up to 25 miles. The aerial equipment is not included in this price, but it can be bought for about $2.50 extra.

a.s.sembling Your Own Receiving Set.--In this chapter we shall go only into the apparatus used for two simple receiving sets, both of which have a _crystal detector_. The first set includes a _double-slide tuning coil_ and the second set employs a _loose-coupled tuning coil_, or _loose coupler_, as it is called for short. For either set you can use a pair of 2,000- or 3,000-ohm head phones.

[Ill.u.s.tration: original Underwood and Underwood. General Pershing Listening In.]

The Crystal Detector.--A crystal detector consists of: (1) _the frame_, (2) _the crystal_, and (3) _the wire point_. There are any number of different designs for frames, the idea being to provide a device that will (a) hold the sensitive crystal firmly in place, and yet permit of its removal, (b) to permit the _wire point_, or _electrode_, to be moved in any direction so that the free point of it can make contact with the most sensitive spot on the crystal and (c) to vary the pressure of the wire on the crystal.

A simple detector frame is shown in the cross-section at A in Fig. 10; the crystal, which may be _galena_, _silicon_ or _iron pyrites_, is held securely in a holder while the _phosphor-bronze wire point_ which makes contact with it, is fixed to one end of a threaded rod on the other end of which is a k.n.o.b. This rod screws into and through a sleeve fixed to a ball that sets between two bra.s.s standards and this permits an up and down or a side to side adjustment of the metal point while the pressure of it on the crystal is regulated by the screw.

[Ill.u.s.tration: (A) Fig. 10.--Cross Section of Crystal Detector.]

[Ill.u.s.tration: (B) Fig. 10.--The Crystal Detector Complete.]

A crystal of this kind is often enclosed in a gla.s.s cylinder and this makes it retain its sensitiveness for a much longer time than if it were exposed to dust and moisture. An upright type of this detector can be bought for $2.25, while a horizontal type, as shown at B, can be bought for $2.75. Galena is the crystal that is generally used, for, while it is not quite as sensitive as silicon and iron pyrites, it is easier to obtain a sensitive piece.

The Tuning Coil.--It is with the tuning coil that you _tune in_ and _tune out_ different stations and this you do by sliding the contacts to and fro over the turns of wire; in this way you vary the _inductance_ and _capacitance_, that is, the _constants_ of the receiving circuits and so make them receive _electric waves_, that is, wireless waves, of different lengths.

The Double Slide Tuning Coil.--With this tuning coil you can receive waves from any station up to 1,000 meters in length. One of the ends of the coil of wire connects with the binding post marked _a_ in Fig.

11, and the other end connects with the other binding post marked _b_, while one of the sliding contacts is connected to the binding post _c_, and the _other sliding contact_ is connected with the binding post _d_.

[Ill.u.s.tration: (A) Fig. 11.--Schematic Diagram of Double Slide Tuning Coil.]

[Ill.u.s.tration: (B) Fig. 11.--Double Slide Tuning Coil Complete.]

When connecting in the tuning coil, only the post _a_ or the post _b_ is used as may be most convenient, but the other end of the wire which is connected to a post is left free; just bear this point in mind when you come to connect the tuning coil up with the other parts of your receiving set. The tuning coil is shown complete at B and it costs $3.00 or $4.00. A _triple slide_ tuning coil constructed like the double slide tuner just described, only with more turns of wire on it, makes it possible to receive wave lengths up to 1,500 meters. It costs about $6.00.

The Loose Coupled Tuning Coil.--With a _loose coupler_, as this kind of a tuning coil is called for short, very _selective tuning_ is possible, which means that you can tune in a station very sharply, and it will receive any wave lengths according to size of coils. The primary coil is wound on a fixed cylinder and its inductance is varied by means of a sliding contact like the double slide tuning coil described above. The secondary coil is wound on a cylinder that slides in and out of the primary coil. The inductance of this coil is varied by means of a switch that makes contact with the fixed points, each of which is connected with every twentieth turn of wire as shown in the diagram A in Fig. 12. The loose coupler, which is shown complete at B, costs in the neighborhood of $8.00 or $10.00.

[Ill.u.s.tration: (A) Fig. 12.--Schematic Diagram of Loose Coupler.]

[Ill.u.s.tration: (B) Fig. 12.--Loose Coupler Complete.]

Fixed and Variable Condensers.--You do not require a condenser for a simple receiving set, but if you will connect a _fixed condenser_ across your headphones you will get better results, while a _variable condenser_ connected in the _closed circuit of a direct coupled receiving set_, that is, one where a double slide tuning coil is used, makes it easy to tune very much more sharply; a variable condenser is absolutely necessary where the circuits are _inductively coupled_, that is, where a loose coupled tuner is used.

A fixed condenser consists of a number of sheets of paper with leaves of tin-foil in between them and so built up that one end of every other leaf of tin-foil projects from the opposite end of the paper as shown at A in Fig. 13. The paper and tin-foil are then pressed together and impregnated with an insulating compound. A fixed condenser of the exact capacitance required for connecting across the head phones is mounted in a base fitted with binding posts, as shown at B, and costs 75 cents. (Paper ones 25 cents.)

[Ill.u.s.tration: (A) Fig. 13.--How a Fixed Receiving Condenser is Built up.]

[Ill.u.s.tration: (B) Fig. 13.--The Fixed Condenser Complete.]

[Ill.u.s.tration: (C) and (D) Fig. 13.--The Variable Rotary Condenser.]

A variable condenser, see C, of the rotating type is formed of a set of fixed semi-circular metal plates which are slightly separated from each other and between these a similar set of movable semi-circular metal plates is made to interleave; the latter are secured to a shaft on the top end of which is a k.n.o.b and by turning it the capacitance of the condenser, and, hence, of the circuit in which it is connected, is varied. This condenser, which is shown at D, is made in two sizes, the smaller one being large enough for all ordinary wave lengths while the larger one is for proportionately longer wave lengths. These condensers cost $4.00 and $5.00 respectively.