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.

Assembling 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.

[Illustration: 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 knob. This rod screws into and through a sleeve fixed to a ball that sets between two brass 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.

[Illustration: (A) Fig. 10.--Cross Section of Crystal Detector.]

[Illustration: (B) Fig. 10.--The Crystal Detector Complete.]

A crystal of this kind is often enclosed in a glass 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_.

[Illustration: (A) Fig. 11.--Schematic Diagram of Double Slide Tuning Coil.]

[Illustration: (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.

[Illustration: (A) Fig. 12.--Schematic Diagram of Loose Coupler.]

[Illustration: (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.)

[Illustration: (A) Fig. 13.--How a Fixed Receiving Condenser is Built up.]

[Illustration: (B) Fig. 13.--The Fixed Condenser Complete.]

[Illustration: (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 knob 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.

About Telephone Receivers.--There are a number of makes of head telephone receivers on the market that are designed especially for wireless work. These phones are wound to _resistances_ of from 75 _ohms_ to 8,000 _ohms_, and cost from $1.25 for a receiver without a cord or headband to $15.00 for a pair of phones with a cord and head band. You can get a receiver wound to any resistance in between the above values but for either of the simple receiving sets such as described in this chapter you ought to have a pair wound to at least 2,000 ohms and these will cost you about $5.00. A pair of head phones of this type is shown in Fig. 14.

[Illustration: Fig. 14.--Pair of Wireless Head Phones.]

Connecting Up the Parts--Receiving Set No. 1.--For this set get (1) a _crystal detector_, (2) a _two-slide tuning coil_, (3) a _fixed condenser_, and (4) a pair of 2,000 ohm head phones. Mount the detector on the right-hand side of a board and the tuning coil on the left-hand side. Screw in two binding posts for the cord ends of the telephone receivers at _a_ and _b_ as shown at A in Fig. 15. This done connect one of the end binding posts of the tuning coil with the ground wire and a post of one of the contact slides with the lightning arrester or switch which leads to the aerial wire.

[Illustration: Fig. 15.--Top View of Apparatus Layout for Receiving Set No. 1.]

[Illustration: (B) Fig. 15.--Wiring Diagram for Receiving Set No. 1.]

Now connect the post of the other contact slide to one of the posts of the detector and the other post of the latter with the binding post _a_, then connect the binding post _b_ to the ground wire and solder the joint. Next connect the ends of the telephone receiver cord to the posts _a_ and _b_ and connect a fixed condenser also with these posts, all of which are shown in the wiring diagram at B, and you are ready to adjust the set for receiving.

Receiving Set No. 2.--Use the same kind of a detector and pair of head phones as for _Set No. 1_, but get (1) a _loose coupled tuning coil_, and (2) a _variable condenser_. Mount the loose coupler at the back of a board on the left-hand side and the variable condenser on the right-hand side. Then mount the detector in front of the variable condenser and screw two binding posts, _a_ and _b_, in front of the tuning coil as shown at A in Fig. 16.

[Illustration: Fig. 16.--Top view of Apparatus Layout for Receiving Set No. 2.]

[Illustration: (B) Fig. 16.--Wiring Diagram for Receiving Set No. 2.]

Now connect the post of the sliding contact of the loose coupler with the wire that runs to the lightning switch and thence to the aerial; connect the post of the primary coil, which is the outside coil, with the ground wire; then connect the binding post leading to the switch of the secondary coil, which is the inside coil, with one of the posts of the variable condenser, and finally, connect the post that is joined to one end of the secondary coil with the other post of the variable condenser.

This done, connect one of the posts of the condenser with one of the posts of the detector, the other post of the detector with the binding post _a_, and the post _b_ to the other post of the variable condenser. Next connect a fixed condenser to the binding posts _a_ and _b_ and then connect the telephone receivers to these same posts, all of which is shown in the wiring diagram at B. You are now ready to adjust the instruments. In making the connections use No. 16 or 18 insulated copper wire and scrape the ends clean where they go into the binding posts. See, also, that all of the connections are tight and where you have to cross the wires keep them apart by an inch or so and always cross them at right angles.

Adjusting the No. 1 Set--The Detector.--The first thing to do is to test the detector in order to find out if the point of the contact