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.

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

[Ill.u.s.tration: (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.

[Ill.u.s.tration: 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 pa.s.s 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 post with the lightning switch which connects with the aerial. Connect the post of the tuning coil with one of the end posts of the switch and the clip of the tuning coil with the other and complementary post of the switch. This done, connect one of the opposite end posts of the switch to the post of the receiving tuning coil and connect the sliding contact of the latter with the other and complementary post of the switch as shown in Fig. 26.

[Ill.u.s.tration: Fig. 26.--Wiring Diagram for Complete Sending and Receiving Set No. 1.]

Connecting in the Lightning Switch.--The aerial wire connects with the middle post of the lightning switch, while one of the end posts lead to one of the middle posts of the aerial switch. The other end post of the lightning switch leads to a separate ground outside the building, as the wiring diagrams Figs. 26 and 27 show.

[Ill.u.s.tration: Fig. 27.--Wiring Diagram for Complete Sending and Receiving Set No. 2.]

CHAPTER V

ELECTRICITY SIMPLY EXPLAINED

It is easy to understand how electricity behaves and what it does if you get the right idea of it at the start. In the first place, if you will think of electricity as being a fluid like water its fundamental actions will be greatly simplified. Both water and electricity may be at rest or in motion. When at rest, under certain conditions, either one will develop pressure, and this pressure when released will cause them to flow through their respective conductors and thus produce a current.

Electricity at Rest and in Motion.--Any wire or a conductor of any kind can be charged with electricity, but a Leyden jar, or other condenser, is generally used to hold an electric charge because it has a much larger _capacitance_, as its capacity is called, than a wire.

As a simple a.n.a.logue of a condenser, suppose you have a tank of water raised above a second tank and that these are connected together by means of a pipe with a valve in it, as shown at A in Fig. 28.

[Ill.u.s.tration: Fig. 28.--Water a.n.a.logue for Electric Pressure.]

[Ill.u.s.tration: original Underwood and Underwood. First Wireless College in the World, at Tufts College, Ma.s.s.]

Now if you fill the upper tank with water and the valve is turned off, no water can flow into the lower tank but there is a difference of pressure between them, and the moment you turn the valve on a current of water will flow through the pipe. In very much the same way when you have a condenser charged with electricity the latter will be under _pressure,_ that is, a _difference of potential_ will be set up, for one of the sheets of metal will be charged positively and the other one, which is insulated from it, will be charged negatively, as shown at B. On closing the switch the opposite charges rush together and form a current which flows to and fro between the metal plates.

[Footnote: Strictly speaking it is the difference of potential that sets up the electromotive force.]

The Electric Current and Its Circuit.--Just as water flowing through a pipe has _quant.i.ty_ and _pressure_ back of it and the pipe offers friction to it which tends to hold back the water, so, likewise, does electricity flowing in a circuit have: (1) _quant.i.ty_, or _current strength_, or just _current_, as it is called for short, or _amperage_, and (2) _pressure_, or _potential difference_, or _electromotive force_, or _voltage_, as it is variously called, and the wire, or circuit, in which the current is flowing has (3) _resistance_ which tends to hold back the current.

A definite relation exists between the current and its electromotive force and also between the current, electromotive force and the resistance of the circuit; and if you will get this relationship clearly in your mind you will have a very good insight into how direct and alternating currents act. To keep a quant.i.ty of water flowing in a loop of pipe, which we will call the circuit, pressure must be applied to it and this may be done by a rotary pump as shown at A in Fig. 29; in the same way, to keep a quant.i.ty of electricity flowing in a loop of wire, or circuit, a battery, or other means for generating electric pressure must be used, as shown at B.

[Ill.u.s.tration: Fig. 29.--Water a.n.a.logues for Direct and Alternating Currents.]

If you have a closed pipe connected with a piston pump, as at C, as the piston moves to and fro the water in the pipe will move first one way and then the other. So also when an alternating current generator is connected to a wire circuit, as at D, the current will flow first in one direction and then in the other, and this is what is called an _alternating current_.

Current and the Ampere.--The amount of water flowing in a closed pipe is the same at all parts of it and this is also true of an electric current, in that there is exactly the same quant.i.ty of electricity at one point of the circuit as there is at any other.

The amount of electricity, or current, flowing in a circuit in a second is measured by a unit called the _ampere_, [Footnote: For definition of _ampere_ see _Appendix._] and it is expressed by the symbol I. [Footnote: This is because the letter C is used for the symbol of _capacitance_] Just to give you an idea of the quant.i.ty of current an _ampere_ is we will say that a dry cell when fresh gives a current of about 20 amperes. To measure the current in amperes an instrument called an _ammeter_ is used, as shown at A in Fig. 30, and this is always connected in _series_ with the line, as shown at B.

[Ill.u.s.tration: Fig. 30.--How the Ammeter and Voltmeter are Used.]

Electromotive Force and the Volt.--When you have a pipe filled with water or a circuit charged with electricity and you want to make them flow you must use a pump in the first case and a battery or a dynamo in the second case. It is the battery or dynamo that sets up the electric pressure as the circuit itself is always charged with electricity.

The more cells you connect together in _series_ the greater will be the electric pressure developed and the more current it will move along just as the amount of water flowing in a pipe can be increased by increasing the pressure of the pump. The unit of electromotive force is the _volt_, and this is the electric pressure which will force a current of _1 ampere_ through a resistance of _1 ohm_; it is expressed by the symbol _E_. A fresh dry cell will deliver a current of about 1.5 volts. To measure the pressure of a current in volts an instrument called a _voltmeter_ is used, as shown at C in Fig. 30, and this is always connected across the circuit, as shown at D.

Resistance and the Ohm.--Just as a water pipe offers a certain amount of resistance to the flow of water through it, so a circuit opposes the flow of electricity in it and this is called _resistance_.

Further, in the same way that a small pipe will not allow a large amount of water to flow through it, so, too, a thin wire limits the flow of the current in it.

If you connect a _resistance coil_ in a circuit it acts in the same way as partly closing the valve in a pipe, as shown at A and B in Fig.

31. The resistance of a circuit is measured by a unit called the _ohm_, and it is expressed by the symbol _R_. A No. 10, Brown and Sharpe gauge soft copper wire, 1,000 feet long, has a resistance of about 1 ohm. To measure the resistance of a circuit an apparatus called a _resistance bridge is used_. The resistance of a circuit can, however, be easily calculated, as the following shows.

[Ill.u.s.tration: Fig. 31.--Water Valve a.n.a.logue of Electric Resistance.

A- a valve limits the flow of water. B- a resistance limits the flow of current.]

What Ohm's Law Is.--If, now, (1) you know what the current flowing in a circuit is in _amperes_, and the electromotive force, or pressure, is in _volts_, you can then easily find what the resistance is in _ohms_ of the circuit in which the current is flowing by this formula:

Volts E --------- = Ohms, or --- = R Amperes I

That is, if you divide the current in amperes by the electromotive force in volts the quotient will give you the resistance in ohms.