Thus the high frequency currents which the steel magnet cores of the telephone receiver would choke off are changed by the detector into intermittent direct currents which can flow through the magnet coils of the telephone receiver. Since the telephone receiver chokes off the oscillations, a small condenser can be shunted around it so that a complete closed oscillation circuit is formed and this gives better results.

When the intermittent rectified current flows through the coils of the telephone receiver it energizes the magnet as long as it lasts, when it is de-energized; this causes the soft iron disk, or _diaphragm_ as it is called, which sets close to the ends of the poles of the magnet, to vibrate; and this in turn gives forth sounds such as dots and dashes, speech or music, according to the nature of the electric waves that sent them out at the distant station.

How Receiving Set No. 2 Works.--When the electric oscillations that are set up by the incoming electric waves on the aerial wire surge through the primary coil of the oscillation transformer they produce a magnetic field and as the lines of force of the latter cut the secondary coil, oscillations of the same frequency are set up in it.

The potential (voltage) of these oscillations are, however, _stepped down_ in the secondary coil and, hence, their current strength (amperes) is increased.

The oscillations then flow through the closed circuit where they are rectified by the crystal detector and transformed into sound waves by the telephone receiver as described in connection with _Set No. 1_.

The variable condenser shunted across the closed circuit permits finer secondary tuning to be done than is possible without it. Where you are receiving continuous waves from a wireless telephone transmitter (speech or music) you have to tune sharper than is possible with the tuning coil alone and to do this a variable condenser connected in parallel with the secondary coil is necessary.

CHAPTER VII

MECHANICAL AND ELECTRICAL TUNING

There is a strikingly close resemblance between _sound waves_ and the way they are set up in _the air_ by a mechanically vibrating body, such as a steel spring or a tuning fork, and _electric waves_ and the way they are set up in _the ether_ by a current oscillating in a circuit. As it is easy to grasp the way that sound waves are produced and behave something will be told about them in this chapter and also an explanation of how electric waves are produced and behave and thus you will be able to get a clear understanding of them and of tuning in general.

Damped and Sustained Mechanical Vibrations.--If you will place one end of a flat steel spring in a vice and screw it up tight as shown at A in Fig. 34, and then pull the free end over and let it go it will vibrate to and fro with decreasing amplitude until it comes to rest as shown at B. When you pull the spring over you store up energy in it and when you let it go the stored up energy is changed into energy of motion and the spring moves forth and back, or _vibrates_ as we call it, until all of its stored up energy is spent.

[Ill.u.s.tration: Fig. 34.--Damped and Sustained Mechanical Vibrations.]

If it were not for the air surrounding it and other frictional losses, the spring would vibrate for a very long time as the stored up energy and the energy of motion would practically offset each other and so the energy would not be used up. But as the spring beats the air the latter is sent out in impulses and the conversion of the vibrations of the spring into waves in the air soon uses up the energy you have imparted to it and it comes to rest.

In order to send out _continuous waves_ in the air instead of _damped waves_ as with a flat steel spring you can use an _electric driven tuning fork_, see C, in which an electromagnet is fixed on the inside of the p.r.o.ngs and when this is energized by a battery current the vibrations of the p.r.o.ngs of the fork are kept going, or are _sustained_, as shown in the diagram at D.

Damped and Sustained Electric Oscillations.--The vibrating steel spring described above is a very good a.n.a.logue of the way that damped electric oscillations which surge in a circuit set up and send out periodic electric waves in the ether while the electric driven tuning fork just described is likewise a good a.n.a.logue of how sustained oscillations surge in a circuit and set up and send out continuous electric waves in the ether as the following shows.

Now the inductance and resistance of a circuit such as is shown at A in Fig. 35, slows down, and finally damps out entirely, the electric oscillations of the high frequency currents, see B, where these are set up by the periodic discharge of a condenser, precisely as the vibrations of the spring are damped out by the friction of the air and other resistances that act upon it. As the electric oscillations surge to and fro in the circuit it is opposed by the action of the ether which surrounds it and electric waves are set up in and sent out through it and this transformation soon uses up the energy of the current that flows in the circuit.

[Ill.u.s.tration: Fig. 35.--Damped and Sustained Electric Oscillations.]

To send out _continuous waves_ in the ether such as are needed for wireless telephony instead of _damped waves_ which are, at the present writing, generally used for wireless telegraphy, an _electric oscillation arc_ or a _vacuum tube oscillator_ must be used, see C, instead of a spark gap. Where a spark gap is used the condenser in the circuit is charged periodically and with considerable lapses of time between each of the charging processes, when, of course, the condenser discharges periodically and with the same time element between them.

Where an oscillation arc or a vacuum tube is used the condenser is charged as rapidly as it is discharged and the result is the oscillations are sustained as shown at D.

About Mechanical Tuning.--A tuning fork is better than a spring or a straight steel bar for setting up mechanical vibrations. As a matter of fact a tuning fork is simply a steel bar bent in the middle so that the two ends are parallel. A handle is attached to middle point of the fork so that it can be held easily and which also allows it to vibrate freely, when the ends of the p.r.o.ngs alternately approach and recede from one another. When the p.r.o.ngs vibrate the handle vibrates up and down in unison with it, and imparts its motion to the _sounding box_, or _resonance case_ as it is sometimes called, where one is used.

If, now, you will mount the fork on a sounding box which is tuned so that it will be in resonance with the vibrations of the fork there will be a direct reinforcement of the vibrations when the note emitted by it will be augmented in strength and quality. This is called _simple resonance_. Further, if you mount a pair of forks, each on a separate sounding box, and have the forks of the same size, tone and pitch, and the boxes synchronized, that is, tuned to the same frequency of vibration, then set the two boxes a foot or so apart, as shown at A in Fig. 36, when you strike one of the forks with a rubber hammer it will vibrate with a definite frequency and, hence, send out sound waves of a given length. When the latter strike the second fork the impact of the molecules of air of which the sound waves are formed will set its p.r.o.ngs to vibrating and it will, in turn, emit sound waves of the same length and this is called _sympathetic resonance_, or as we would say in wireless the forks are _in tune_.

[Ill.u.s.tration: Fig. 36.--Sound Wave and Electric Wave Tuned Senders and Receptors. A - variable tuning forks for showing sound wave tuning. B - variable oscillation circuits for showing electric wave tuning.]

Tuning forks are made with adjustable weights on their p.r.o.ngs and by fixing these to different parts of them the frequency with which the forks vibrate can be changed since the frequency varies inversely with the square of the length and directly with the thickness [Footnote: This law is for forks having a rectangular cross-section. Those having a round cross-section vary as the radius.] of the p.r.o.ngs. Now by adjusting one of the forks so that it vibrates at a frequency of, say, 16 per second and adjusting the other fork so that it vibrates at a frequency of, say, 18 or 20 per second, then the forks will not be in tune with each other and, hence, if you strike one of them the other will not respond. But if you make the forks vibrate at the same frequency, say 16, 20 or 24 per second, when you strike one of them the other will vibrate in unison with it.

About Electric Tuning.--Electric resonance and electric tuning are very like those of acoustic resonance and acoustic tuning which I have just described. Just as acoustic resonance may be simple or sympathetic so electric resonance may be simple or sympathetic. Simple acoustic resonance is the direct reinforcement of a simple vibration and this condition is had when a tuning fork is mounted on a sounding box. In simple electric resonance an oscillating current of a given frequency flowing in a circuit having the proper inductance and capacitance may increase the voltage until it is several times greater than its normal value. Tuning the receptor circuits to the transmitter circuits are examples of sympathetic electric resonance. As a demonstration if you have two Leyden jars (capacitance) connected in circuit with two loops of wire (inductance) whose inductance can be varied as shown at B in Fig. 36, when you make a spark pa.s.s between the k.n.o.bs of one of them by means of a spark coil then a spark will pa.s.s in the gap of the other one provided the inductance of the two loops of wire is the same. But if you vary the inductance of the one loop so that it is larger or smaller than that of the other loop no spark will take place in the second circuit.

When a tuning fork is made to vibrate it sends out waves in the air, or sound waves, in all directions and just so when high frequency currents surge in an oscillation circuit they send out waves in the ether, or electric waves, that travel in all directions. For this reason electric waves from a transmitting station cannot be sent to one particular station, though they do go further in one direction than in another, according to the way your aerial wire points.

Since the electric waves travel out in all directions any receiving set properly tuned to the wave length of the sending station will receive the waves and the only limit on your ability to receive from high-power stations throughout the world depends entirely on the wave length and sensitivity of your receiving set. As for tuning, just as changing the length and the thickness of the p.r.o.ngs of a tuning fork varies the frequency with which it vibrates and, hence, the length of the waves it sends out, so, too, by varying the capacitance of the condenser and the inductance of the tuning coil of the transmitter the frequency of the electric oscillations set up in the circuit may be changed and, consequently, the length of the electric waves they send out. Likewise, by varying the capacitance and the inductance of the receptor the circuits can be tuned to receive incoming electric waves of whatever length within the limitation of the apparatus.

CHAPTER VIII

A SIMPLE VACUUM TUBE DETECTOR RECEIVING SET

While you can receive dots and dashes from spark wireless telegraph stations and hear spoken words and music from wireless telephone stations with a crystal detector receiving set such as described in Chapter III, you can get stations that are much farther away and hear them better with a _vacuum tube detector_ receiving set.

Though the vacuum tube detector requires two batteries to operate it and the receiving circuits are somewhat more complicated than where a crystal detector is used still the former does not have to be constantly adjusted as does the latter and this is another very great advantage. Taken all in all the vacuum tube detector is the most sensitive and the most satisfactory of the detectors that are in use at the present time.

Not only is the vacuum tube a detector of electric wave signals and speech and music but it can also be used to _amplify_ them, that is, to make them stronger and, hence, louder in the telephone receiver and further its powers of amplification are so great that it will reproduce them by means of a _loud speaker_, just as a horn amplifies the sounds of a phonograph reproducer, until they can be heard by a room or an auditorium full of people. There are two general types of loud speakers, though both use the principle of the telephone receiver. The construction of these loud speakers will be fully described in a later chapter.

a.s.sembled Vacuum Tube Receiving Sets.--You can buy a receiving set with a vacuum tube detector from the very simplest type, which is described in this chapter, to those that are provided with _regenerative circuits_ and _amplifying_ tubes or both, which we shall describe in later chapters, from dealers in electrical apparatus generally. While one of these sets costs more than you can a.s.semble a set for yourself, still, especially in the beginning, it is a good plan to buy an a.s.sembled one for it is fitted with a _panel_ on which the adjusting k.n.o.bs of the rheostat, tuning coil and condenser are mounted and this makes it possible to operate it as soon as you get it home and without the slightest trouble on your part.

You can, however, buy all the various parts separately and mount them yourself. If you want the receptor simply for receiving then it is a good scheme to have all of the parts mounted in a box or enclosed case, but if you want it for experimental purposes then the parts should be mounted on a base or a panel so that all of the connections are in sight and accessible.

A Simple Vacuum Tube Receiving Set.--For this set you should use: (1) a _loose coupled tuning coil,_ (2) a _variable condenser,_ (3) a _vacuum tube detector,_ (4) an A or _storage battery_ giving 6 volts, (5) a B or _dry cell battery_ giving 22-1/2 volts, (6) a _rheostat_ for varying the storage battery current, and (7) a pair of 2,000-ohm _head telephone receivers_. The loose coupled tuning coil, the variable condenser and the telephone receivers are the same as those described in Chapter III.

The Vacuum Tube Detector. With Two Electrodes.--A vacuum tube in its simplest form consists of a gla.s.s bulb like an incandescent lamp in which a _wire filament_ and a _metal plate_ are sealed as shown in Fig. 37, The air is then pumped out of the tube and a vacuum left or after it is exhausted it is filled with nitrogen, which cannot burn.

[Ill.u.s.tration: Fig. 37.--Two Electrode Vacuum Tube Detectors.]

When the vacuum tube is used as a detector, the wire filament is heated red-hot and the metal plate is charged with positive electricity though it remains cold. The wire filament is formed into a loop like that of an incandescent lamp and its outside ends are connected with a 6-volt storage battery, which is called the A battery; then the + or _positive_ terminal of a 22-1/2 volt dry cell battery, called the B battery, is connected to the metal plate while the - or _negative_ terminal of the battery is connected to one of the terminals of the wire filament. The diagram, Fig. 37, simply shows how the two electrode vacuum tube, the A or dry battery, and the B or storage battery are connected up.

Three Electrode Vacuum Tube Detector.--The three electrode vacuum tube detector shown at A in Fig. 38, is much more sensitive than the two electrode tube and has, in consequence, all but supplanted it. In this more recent type of vacuum tube the third electrode, or _grid_, as it is called, is placed between the wire filament and the metal plate and this allows the current to be increased or decreased at will to a very considerable extent.

[Ill.u.s.tration: Fig. 38.--Three Electrode Vacuum Tube Detector and Battery Connections.]

The way the three electrode vacuum tube detector is connected with the batteries is shown at B. The plate, the A or dry cell battery and one terminal of the filament are connected in _series_--that is, one after the other, and the ends of the filament are connected to the B or storage battery. In a.s.sembling a receiving set you must, of course, have a socket for the vacuum tube. A vacuum tube detector costs from $5.00 to $6.00.

The Dry Cell and Storage Batteries.--The reason that a storage battery is used for heating the filament of the vacuum tube detector is because the current delivered is constant, whereas when a dry cell battery is used the current soon falls off and, hence, the heat of the filament gradually grows less. The smallest A or 6 volt storage battery on the market has a capacity of 20 to 40 ampere hours, weighs 13 pounds and costs about $10.00. It is shown at A in Fig. 39. The B or dry cell battery for the vacuum tube plate circuit that gives 22-1/2 volts can be bought already a.s.sembled in sealed boxes. The small size is fitted with a pair of terminals while the larger size is provided with _taps_ so that the voltage required by the plate can be adjusted as the proper operation of the tube requires careful regulation of the plate voltage. A dry cell battery for a plate circuit is shown at B.

[Ill.u.s.tration: Fig. 39.--A and B Batteries for Vacuum Tube Detectors.]

The Filament Rheostat.--An adjustable resistance, called a _rheostat_, must be used in the filament and storage battery circuit so that the current flowing through the filament can be controlled to a nicety.

The rheostat consists of an insulating and a heat resisting form on which is wound a number of turns of resistance wire. A movable contact arm that slides over and presses on the turns of wire is fixed to the k.n.o.b on top of the rheostat. A rheostat that has a resistance of 6 ohms and a current carrying capacity of 1.5 amperes which can be mounted on a panel board is the right kind to use. It is shown at A and B in Fig. 40 and costs $1.25.

[Ill.u.s.tration: Fig. 40.--Rheostat for the A or Storage Battery Current.]

a.s.sembling the Parts.--Begin by placing all of the separate parts of the receiving set on a board or a base of other material and set the tuning coil on the left hand side with the adjustable switch end toward the right hand side so that you can reach it easily. Then set the variable condenser in front of it, set the vacuum tube detector at the right hand end of the tuning coil and the rheostat in front of the detector. Place the two sets of batteries back of the instruments and screw a couple of binding posts _a_ and _b_ to the right hand lower edge of the base for connecting in the head phones all of which is shown at A in Fig. 41.

[Ill.u.s.tration: (A) Fig. 41.--Top View of Apparatus Layout for a Vacuum Tube Detector Receiving Set.]

[Ill.u.s.tration: (B) Fig. 41.--Wiring Diagram of a Simple Vacuum Tube Receiving Set.]

Connecting Up the Parts.--To wire up the different parts begin by connecting the sliding contact of the primary coil of the loose coupled tuning coil (this you will remember is the outside one that is wound with fine wire) to the upper post of the lightning switch and connect one terminal of this coil with the water pipe. Now connect the free end of the secondary coil of the tuning coil (this is the inside coil that is wound with heavy wire) to one of the binding posts of the variable condenser and connect the movable contact arm of the adjustable switch of the primary of the tuning coil with the other post of the variable condenser.