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- The Radio Amateur's Hand Book - 10/44 -


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.

[Illustration: 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.

The Wireless Key.--For this transmitting set a standard wireless key should be used as shown at B. It is made about the same as a regular telegraph key but it is much heavier, the contact points are larger and instead of the current being led through the bearings as in an ordinary key, it is carried by heavy conductors directly to the contact points. This key is made in three sizes and the first will carry a current of 5 _amperes_[Footnote: See _Appendix_ for definition.] and costs $4.00, the second will carry a current of 10 amperes and costs $6.50, while the third will carry a current of 20 amperes and costs $7.50.

The Spark Gap.--Either a fixed, a rotary, or a quenched spark gap can be used with this set, but the former is seldom used except with spark-coil sets, as it is very hard to keep the sparks from arcing when large currents are used. A rotary spark gap comprises a wheel, driven by a small electric motor, with projecting plugs, or electrodes, on it and a pair of stationary plugs on each side of the wheel as shown at C. The number of sparks per second can be varied by changing the speed of the wheel and when it is rotated rapidly it sends out signals of a high pitch which are easy to read at the receiving end. A rotary gap with a 110-volt motor costs about $25.00.

A quenched spark gap not only eliminates the noise of the ordinary gap but, when properly designed, it increases the range of an induction coil set some 200 per cent. A 1/4 kilowatt quenched gap costs $10.00. [Footnote: See Appendix for definition.]

The High Tension Condenser.--Since, if you are an amateur, you can only send out waves that are 200 meters in length, you can only use a condenser that has a capacitance of .007 _microfarad_. [Footnote: See Appendix for definition.] A sectional high tension condenser like the one described in connection with _Set No. 1_ can be used with this set but it must have a capacitance of not more than .007 microfarad. A condenser of this value for a 1/4-kilowatt transformer costs $7.00; for a 1/2-kilowatt transformer $14.00, and for a 1-kilowatt transformer $21.00. See E, Fig. 19.

The Oscillation Transformer.--With an oscillation transformer you can tune much more sharply than with a single inductance coil tuner. The primary coil is formed of 6 turns of copper strip, or No. 9 copper wire, and the secondary is formed of 9 turns of strip, or wire. The primary coil, which is the outside coil, is hinged to the base and can be raised or lowered like the lid of a box. When it is lowered the primary and secondary coils are in the same plane and when it is raised the coils set at an angle to each other. It is shown at D and costs $5.00.

Connecting Up the Apparatus. For Alternating Current.--Screw the key to the table about the middle of it and near the front edge; place the high tension condenser back of it and the oscillation transformer back of the latter; set the alternating current transformer to the left of the oscillation transformer and place the rotary or quenched spark gap in front of it.

Now bring a pair of _No. 12_ or _14_ insulated wires from the 110 volt lighting leads and connect them with a single-throw, double-pole switch; connect one pole of the switch with one of the posts of the primary coil of the alternating power transformer and connect the other post of the latter with one of the posts of your key, and the other post of this with the other pole of the switch. Now connect the motor of the rotary spark gap to the power circuit and put a single-pole, single-throw switch in the motor circuit, all of which is shown at A in Fig. 22.

[Illustration: (A) Fig. 22.--Top View of Apparatus Layout for Sending Set No. 2.]

[Illustration: (B) Fig. 22.--Wiring Diagram for Sending Set No. 2.]

Next connect the posts of the secondary coil to the posts of the rotary or quenched spark gap and connect one post of the latter to one post of the condenser, the other post of this to the post of the primary coil of the oscillation transformer, which is the inside coil, and the clip of the primary coil to the other spark gap post. This completes the closed oscillation circuit. Finally connect the post of the secondary coil of the oscillation transformer to the ground and the clip of it to the wire leading to the aerial when you are ready to tune the set. A wiring diagram of the connections is shown at B.

For Direct Current.--Where you have 110 volt direct current you must connect in an electrolytic interrupter. This interrupter, which is shown at A and B in Fig. 23, consists of (1) a jar filled with a solution of 1 part of sulphuric acid and 9 parts of water, (2) a lead electrode having a large surface fastened to the cover of surface that sets in a porcelain sleeve and whose end rests on the bottom of the jar.

[Illustration: Fig. 23.--Using 110 Volt Direct Current with an Alternating Current Transformer.]

When these electrodes are connected in series with the primary of a large spark coil or an alternating current transformer, see C, and a direct current of from 40 to 110 volts is made to pass through it, the current is made and broken from 1,000 to 10,000 times a minute. By raising or lowering the sleeve, thus exposing more or less of the platinum, or alloy point, the number of interruptions per minute can be varied at will. As the electrolytic interrupter will only operate in one direction, you must connect it with its platinum, or alloy anode, to the + or _positive_ power lead and the lead cathode to the - or _negative_ power lead. You can find out which is which by connecting in the interrupter and trying it, or you can use a polarity indicator. An electrolytic interrupter can be bought for as little as $3.00.

How to Adjust Your Transmitter. Tuning With a Hot Wire Ammeter.--A transmitter can be tuned in two different ways and these are: (1) by adjusting the length of the spark gap and the tuning coil so that the greatest amount of energy is set up in the oscillating circuits, and (2) by adjusting the apparatus so that it will send out waves of a given length.

To adjust the transmitter so that the circuits will be in tune you should have a _hot wire ammeter_, or radiation ammeter, as it is called, which is shown in Fig. 24. It consists of a thin platinum wire through which the high-frequency currents surge and these heat it; the expansion and contraction of the wire moves a needle over a scale marked off into fractions of an ampere. When the spark gap and tuning coil of your set are properly adjusted, the needle will swing farthest to the right over the scale and you will then know that the aerial wire system, or open oscillation circuit, and the closed oscillation circuit are in tune and radiating the greatest amount of energy.

[Illustration: Fig. 24.--Principle of the Hot Wire Ammeter.]

To Send Out a 200 Meter Wave Length.--If you are using a condenser having a capacitance of .007 microfarad, which is the largest capacity value that the Government will allow an amateur to use, then if you have a hot wire ammeter in your aerial and tune the inductance coil or coils until the ammeter shows the largest amount of energy flowing through it you will know that your transmitter is tuned and that the aerial is sending out waves whose length is 200 meters. To tune to different wave lengths you must have a _wave-meter_.

The Use of the Aerial Switch.--Where you intend to install both a transmitter and a receptor you will need a throwover switch, or _aerial switch_, as it is called. An ordinary double-pole, double-throw switch, as shown at A in Fig. 25, can be used, or a switch made especially for the purpose as at B is handier because the arc of the throw is much less.

[Illustration: Fig. 25.--Kinds of Aerial Switches.]

Aerial Switch for a Complete Sending and Receiving Set.--You can buy a double-pole, double-throw switch mounted on a porcelain base for about 75 cents and this will serve for _Set No. 1_. Screw this switch on your table between the sending and receiving sets and then connect one of the middle posts of it with the ground wire and the other middle


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