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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Fig. 15.--Top View of Apparatus Layout for Receiving Set No. 1.]

[Ill.u.s.tration: (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.

[Ill.u.s.tration: Fig. 16.--Top view of Apparatus Layout for Receiving Set No. 2.]

[Ill.u.s.tration: (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 sc.r.a.pe 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 wire is on a sensitive spot of the crystal. To do this you need a _buzzer_, a _switch_ and a _dry cell_. An electric bell from which the gong has been removed will do for the buzzer, but you can get one that is made specially for the purpose, for 75 cents, which gives out a clear, high-pitched note that sounds like a high-power station.

Connect one of the binding posts of the buzzer with one post of the switch, the other post of the latter with the zinc post of the dry cell and the carbon post of this to the other post of the buzzer. Then connect the post of the buzzer that is joined to the vibrator, to the ground wire as shown in the wiring diagram, Fig. 17. Now close the switch of the buzzer circuit, put on your head phones, and move the wire point of the detector to various spots on the crystal until you hear the sparks made by the buzzer in your phones.

[Ill.u.s.tration: Fig. 17.--Adjusting the Receiving Set.]

Then vary the pressure of the point on the crystal until you hear the sparks as loud as possible. After you have made the adjustment open the switch and disconnect the buzzer wire from the ground wire of your set. This done, be very careful not to jar the detector or you will throw it out of adjustment and then you will have to do it all over again. You are now ready to tune the set with the tuning coil and listen in.

The Tuning Coil.--To tune this set move the slide A of the double-slide tuner, see B in Fig. 15, over to the end of the coil that is connected with the ground wire and the slide B near the opposite end of the coil, that is, the one that has the free end. Now move the slide A toward the B slide and when you hear the dots and dashes, or speech or music, that is coming in as loud as you can move the B slide toward the A slide until you hear still more loudly. A very few trials on your part and you will be able to tune in or tune out any station you can hear, if not too close or powerful.

[Ill.u.s.tration: original Underwood and Underwood. The World's Largest Radio Receiving Station. Owned by the Radio Corporation of America at Rocky Point near Point Jefferson, L.I.]

Adjusting the No. 2 Set.--First adjust the crystal detector with the buzzer set as described above with _Set No. 1,_ then turn the k.n.o.b of your variable condenser so that the movable plates are just half-way in, pull the secondary coil of your loose-coupled tuner half way out; turn the switch lever on it until it makes a contact with the middle contact point and set the slider of the primary coil half way between the ends.

Now listen in for telegraphic signals or telephonic speech or music; when you hear one or the other slide the secondary coil in and out of the primary coil until the sounds are loudest; now move the contact switch over the points forth and back until the sounds are still louder, then move the slider to and fro until the sounds are yet louder and, finally, turn the k.n.o.b of the condenser until the sounds are clear and crisp. When you have done all of these things you have, in the parlance of the wireless operator, _tuned in_ and you are ready to receive whatever is being sent.

CHAPTER IV

SIMPLE TELEGRAPH SENDING SETS

A wireless telegraph transmitting set can be installed for a very small amount of money provided you are content with one that has a limited range. Larger and better instruments can, of course, be had for more money, but however much you are willing to spend still you are limited in your sending radius by the Government's rules and regulations. The best way, and the cheapest in the end, to install a telegraph set is to buy the separate parts and hook them up yourself.

The usual type of wireless telegraph transmitter employs a _disruptive discharge,_ or _spark,_ as it is called, for setting up the oscillating currents in the aerial wire system and this is the type of apparatus described in this chapter. There are two ways to set up the sparks and these are: (1) with an _induction coil,_ or _spark-coil,_ as it is commonly called, and (2) with an _alternating current transformer_, or _power transformer_, as it is sometimes called. Where you have to generate the current with a battery you must use a spark coil, but if you have a 110-volt direct or alternating lighting current in your home you can use a transformer which will give you more power.

A Cheap Transmitting Set (No. 1).--For this set you will need: (1) a _spark-coil_, (2) a _battery_ of dry cells, (3) a _telegraph key_, (4) a _spark gap_, (5) a _high-tension condenser_, and (6) an _oscillation transformer_. There are many different makes and styles of these parts but in the last a.n.a.lysis all of them are built on the same underlying bases and work on the same fundamental principles.

The Spark-Coil.--Spark coils for wireless work are made to give sparks from 1/4 inch in length up to 6 inches in length, but as a spark coil that gives less than a 1-inch spark has a very limited output it is best to get a coil that gives at least a 1-inch spark, as this only costs about $8.00, and if you can get a 2- or a 4-inch spark coil so much the better. There are two general styles of spark coils used for wireless and these are shown at A and B in Fig. 18.

[Ill.u.s.tration: (A) and (B) Fig. 18.--Types of Spark Coils for Set. No.

1.]

[Ill.u.s.tration: (C) Fig. 18.--Wiring Diagram of Spark Coil]

A spark coil of either style consists of (_a_) a soft _iron core_ on which is wound (_b_) a couple of layers of heavy insulated wire and this is called the _primary coil_, (_c_) while over this, but insulated from it, is wound a large number of turns of very fine insulated copper wire called the _secondary coil_; (d) an _interrupter_, or _vibrator_, as it is commonly called, and, finally, (e) a _condenser_. The core, primary and secondary coils form a unit and these are set in a box or mounted on top of a hollow wooden base.

The condenser is placed in the bottom of the box, or on the base, while the vibrator is mounted on one end of the box or on top of the base, and it is the only part of the coil that needs adjusting.

The vibrator consists of a stiff, flat spring fixed at one end to the box or base while it carries a piece of soft iron called an _armature_ on its free end and this sets close to one end of the soft iron core.

Insulated from this spring is a standard that carries an adjusting screw on the small end of which is a platinum point and this makes contact with a small platinum disk fixed to the spring. The condenser is formed of alternate sheets of paper and tinfoil built up in the same fashion as the receiving condenser described under the caption of _Fixed and Variable Condensers_, in Chapter III.

The wiring diagram C shows how the spark coil is wired up. One of the battery binding posts is connected with one end of the primary coil while the other end of the latter which is wound on the soft iron core connects with the spring of the vibrator. The other battery binding post connects with the standard that supports the adjusting screw. The condenser is shunted across the vibrator, that is, one end of the condenser is connected with the spring and the other end of the condenser is connected with the adjusting screw standard. The ends of the secondary coil lead to two binding posts, which are usually placed on top of the spark coil and it is to these that the spark gap is connected.

The Battery.--This can be formed of dry cells or you can use a storage battery to energize your coil. For all coils that give less than a 1-inch spark you should use 5 dry cells; for 1-and 2-inch spark coils use 6 or 8 dry cells, and for 3 to 4-inch spark coils use 8 to 10 dry cells. The way the dry cells are connected together to form a battery will be shown presently. A dry cell is shown at A in Fig, 19.

[Ill.u.s.tration: Fig. 19.--Other parts for Transmitting Set No. 1]

The Telegraph Key.--You can use an ordinary Morse telegraph key for the sending set and you can get one with a j.a.panned iron base for $1.50 (or better, one made of bra.s.s and which has 1/8-inch silver contact points for $3.00. A key of the latter kind is shown at B).

The Spark gap.--It is in the _spark gap_ that the high tension spark takes place. The apparatus in which the spark takes place is also called the _spark gap_. It consists of a pair of zinc plugs, called _electrodes_, fixed to the ends of a pair of threaded rods, with k.n.o.bs on the other ends, and these screw into and through a pair of standards as shown at _c_. This is called a _fixed_, or _stationary spark gap_ and costs about $1.00.

The Tuning Coil.--The _transmitting inductance_, or _sending tuning coil_, consists of 20 to 30 turns of _No. 8 or 9_ hard drawn copper wire wound on a slotted insulated form and mounted on a wooden base.

It is provided with _clips_ so that you can cut in and cut out as many turns of wire as you wish and so tune the sending circuits to send out whatever wave length you desire. It is shown at _d_, and costs about $5.00. See also _Oscillation Transformer_, page 63 [Chapter IV].

The High Tension Condenser.--High tension condensers, that is, condensers which will stand up under _high potentials_, or electric pressures, can be bought in units or sections. These condensers are made up of thin bra.s.s plates insulated with a special compound and pressed into a compact form. The _capacitance_ [Footnote: This is the capacity of the condenser.] of one section is enough for a transmitting set using a spark coil that gives a 2 inch spark or less and two sections connected together should be used for coils giving from 2 to 4 inch sparks. It is shown at _e_.

Connecting Up the Apparatus.--Your sending set should be mounted on a table, or a bench, where it need not be moved. Place the key in about the middle of the table and down in front, and the spark coil to the left and well to the back but so that the vibrator end will be to the right, as this will enable you to adjust it easily. Place the battery back of the spark coil and the tuning coil (oscillation transformer) to the right of the spark coil and back of the key, all of which is shown in the layout at A in Fig. 20.

[Ill.u.s.tration: (A) Fig. 20.--Top View of Apparatus Layout for Sending Set No. 1.]

[Ill.u.s.tration: (B) Fig. 20.--Wiring of Diagram for Sending Set No. 1.]

For the _low voltage circuit_, that is the battery circuit, use _No.

12_ or _14_ insulated copper wire. Connect all of the dry cells together in _series_, that is, connect the zinc of one cell with the carbon of the next and so on until all of them are connected up. Then connect the carbon of the end cell with one of the posts of the key, the zinc of the other end cell with one of the primary posts of the spark coil and the other primary post of the spark coil with the other post of the key, when the primary circuit will be complete.

For the _high tension circuits_, that is, the _oscillation circuits_, you may use either bare or insulated copper wire but you must be careful that they do not touch the table, each other, or any part of the apparatus, except, of course, the posts they are connected with.

Connect one of the posts of the secondary coil of the spark coil with one of the posts of the spark gap, and the other post with one of the posts of the condenser; then connect the other post of the condenser with the lower spring clip of the tuning coil and also connect this clip with the ground. This done, connect the middle spring clip with one of the posts of the spark gap, and, finally, connect the top clip with the aerial wire and your transmitting set is ready to be tuned. A wiring diagram of the connections is shown at B. As this set is tuned in the same way as _Set No. 2_ which follows, you are referred to the end of this chapter.

A Better Transmitting Set (No. 2).--The apparatus for this set includes: (1) an _alternating current transformer_, (2) a _wireless telegraph key_, (3) a _fixed_, a _rotary_, or a _quenched spark gap_, (4) a _condenser_, and (5) an _oscillation transformer_. If you have a 110 volt direct lighting current in your home instead of 110 volt alternating current, then you will also need (6) an _electrolytic interrupter_, for in this case the primary circuit of the transformer must be made and broken rapidly in order to set up alternating currents in the secondary coil.

The Alternating Current Transformer.--An alternating current, or power, transformer is made on the same principle as a spark coil, that is, it has a soft iron core, a primary coil formed of a couple of layers of heavy wire, and a secondary coil wound up of a large number of turns of very fine wire. Unlike the spark coil, however, which has an _open magnetic core_ and whose secondary coil is wound on the primary coil, the transformer has a _closed magnetic core_, with the primary coil wound on one of the legs of the core and the secondary wound on the other leg. It has neither a vibrator nor a condenser. A plain transformer is shown at A in Fig. 21.

[Ill.u.s.tration: Fig. 21.--Parts for Transmitting Set No. 2.]

A transformer of this kind can be bought either (a) _unmounted_, that is, just the bare transformer, or (b) _fully mounted_, that is, fitted with an iron stand, mounted on an insulating base on which are a pair of primary binding posts, while the secondary is provided with a _safety spark gap_. There are three sizes of transformers of this kind made and they are rated at 1/4, 1/2 and 1 kilowatt, respectively, they deliver a secondary current of 9,000, 11,000 and 25,000 volts, according to size, and cost $16.00, $22.00 and $33.00 when fully mounted; a reduction of $3.00, $4.00 and $5.00 is made when they are unmounted. All of these transformers operate on 110 volt, 60 cycle current and can be connected directly to the source of alternating current.