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 prongs and when this is energized by a battery current the vibrations of the prongs 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 analogue 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 analogue 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.
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 prongs alternately approach and recede from one another. When the prongs 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 prongs 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.
Tuning forks are made with adjustable weights on their prongs 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 prongs. 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 pass between the knobs of one of them by means of a spark coil then a spark will pass 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 prongs 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.
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.
Assembled 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 assemble a set for yourself, still, especially in the beginning, it is a good plan to buy an assembled one for it is fitted with a panel on which the adjusting knobs 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 glass 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.
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.
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 assembling 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 assembled 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.
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 knob 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.
Assembling 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.
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.
Next connect the grid of the vacuum tube to one of the posts of the condenser and then connect the plate of the tube to the carbon terminal of the B or dry cell battery which is the + or positive pole and connect the zinc terminal of the - or negative pole to the binding post a, connect the post b to the other side of the variable condenser and then connect the terminals of the head phones to the binding posts a and b. Whatever you do be careful not to get the plate connections of the battery reversed.
Now connect one of the posts of the rheostat to one terminal of the filament and the other terminal of the filament to the - or negative terminal of the A or storage battery and the + or positive terminal of the A or storage battery to the other post of the rheostat. Finally connect the + or positive terminal of the A or storage battery with the wire that runs from the head phones to the variable condenser, all of which is shown in the wiring diagram at B in Fig. 41.
Adjusting the Vacuum Tube Detector Receiving Set.--A vacuum tube detector is tuned exactly in the same way as the Crystal Detector Set No. 2 described in Chapter III, in-so-far as the tuning coil and variable condenser are concerned. The sensitivity of the vacuum tube detector receiving set and, hence, the distance over which signals and other sounds can be heard depends very largely on the sensitivity of the vacuum tube itself and this in turn depends on: (1) the right amount of heat developed by the filament, or filament brilliancy as it is called, (2) the right amount of voltage applied to the plate, and (3) the extent to which the tube is exhausted where this kind of a tube is used.
To vary the current flowing from the A or storage battery through the filament so that it will be heated to the right degree you adjust the rheostat while you are listening in to the signals or other sounds. By carefully adjusting the rheostat you can easily find the point at which it makes the tube the most sensitive. A rheostat is also useful in that it keeps the filament from burning out when the current from the battery first flows through it. You can very often increase the sensitiveness of a vacuum tube after you have used it for a while by recharging the A or storage battery.
The degree to which a vacuum tube has been exhausted has a very pronounced effect on its sensitivity. The longer the tube is used the lower its vacuum gets and generally the less sensitive it becomes. When this takes place (and you can only guess at it) you can very often make it more sensitive by warming it over the flame of a candle. Vacuum tubes having a gas content (in which case they are, of course, no longer vacuum tubes in the strict sense) make better detectors than tubes from which the air has been exhausted and which are sealed off in this evacuated condition because their sensitiveness is not dependent on the degree of vacuum as in the latter tubes. Moreover, a tube that is completely exhausted costs more than one that is filled with gas.
The reason a vacuum tube detector is more sensitive than a crystal detector is because while the latter merely rectifies the oscillating current that surges in the receiving circuits, the former acts as an amplifier at the same time. The vacuum tube can be used as a separate amplifier in connection with either: (1) a crystal detector or (2) a vacuum tube detector, and (a) it will amplify either the radio frequency currents, that is the high frequency oscillating currents which are set up in the oscillation circuits or (b) it will amplify the audio frequency currents, that is, the low frequency alternating currents that flow through the head phone circuit.
To use the amplified radio frequency oscillating currents or amplified audio frequency alternating currents that are set up by an amplifier tube either a high resistance, called a grid leak, or an amplifying transformer, with or without an iron core, must be connected with the plate circuit of the first amplifier tube and the grid circuit of the next amplifier tube or detector tube, or with the wire point of a crystal detector. Where two or more amplifier tubes are coupled together in this way the scheme is known as cascade amplification.
Where either a radio frequency transformer, that is one without the iron core, or an audio frequency transformer, that is one with the iron core, is used to couple the amplifier tube circuits together better results are obtained than where a high resistance grid leak is used, but the amplifying tubes have to be more carefully shielded from each other or they will react and set up a howling noise in the head phones. On the other hand grid leaks cost less but they are more troublesome to use as you have to find out for yourself the exact resistance value they must have and this you can do only by testing them out.
A Grid Leak Amplifier Receiving Set. With Crystal Detector.--The apparatus you need for this set includes: (1) a loose coupled tuning coil, (2) a variable condenser, (3) two fixed condensers, (4) a crystal detector, or better a vacuum tube detector, (5) an A or 6 volt storage battery, (6) a rheostat, (7) a B or 22-1/2 volt dry cell battery, (8) a fixed resistance unit, or leak grid as it is called, and (9) a pair of head-phones. The tuning coil, variable condenser, fixed condensers, crystal detectors and head-phones are exactly the same as those described in Set No. 2 in Chapter III. The A and B batteries are exactly the same as those described in Chapter VIII. The vacuum tube amplifier and the grid leak are the only new pieces of apparatus you need and not described before.
The Vacuum Tube Amplifier.--This consists of a three electrode vacuum tube exactly like the vacuum tube detector described in Chapter VIII and pictured in Fig. 38, except that instead of being filled with a non-combustible gas it is evacuated, that is, the air has been completely pumped out of it. The gas filled tube, however, can be used as an amplifier and either kind of tube can be used for either radio frequency or audio frequency amplification, though with the exhausted tube it is easier to obtain the right plate and filament voltages for good working.
The Fixed Resistance Unit, or Grid Leak.--Grid leaks are made in different ways but all of them have an enormously high resistance. One way of making them consists of depositing a thin film of gold on a sheet of mica and placing another sheet of mica on top to protect it the whole being enclosed in a glass tube as shown at A in Fig. 42. These grid leaks are made in units of from 50,000 ohms (.05 megohm) to 5,000,000 ohms (5 megohms) and cost from $1 to $2.
As the value of the grid leak you will need depends very largely upon the construction of the different parts of your receiving set and on the kind of aerial wire system you use with it you will have to try out various resistances until you hit the right one. The resistance that will give the best results, however, lies somewhere between 500,000 ohms (1/2 a megohm) and 3,000,000 ohms (3 megohms) and the only way for you to find this out is to buy 1/2, 1 and 2 megohm grid leak resistances and connect them up in different ways, as shown at B, until you find the right value.
Assembling the Parts for a Crystal Detector Set.--Begin by laying the various parts out on a base or a panel with the loose coupled tuning coil on the left hand side, but with the adjustable switch of the secondary coil on the right hand end or in front according to the way it is made. Then place the variable condenser, the rheostat, the crystal detector and the binding posts for the head phones in front of and in a line with each other. Set the vacuum tube amplifier back of the rheostat and the A and B batteries back of the parts or in any other place that may be convenient. The fixed condensers and the grid leak can be placed anywhere so that it will be easy to connect them in and you are ready to wire up the set.
Connecting Up the Parts for a Crystal Detector.--First connect the sliding contact of the primary of the tuning coil to the leading-in wire and one of the end wires of the primary to the water pipe, as shown in Fig. 43. Now connect the adjustable arm that makes contact with one end of the secondary of the tuning coil to one of the posts of the variable condenser; then connect the other post of the latter with a post of the fixed condenser and the other post of this with the grid of the amplifying tube.
Connect the first post of the variable condenser to the + or positive electrode of the A battery and its - or negative electrode with the rotating contact arm of the rheostat. Next connect one end of the resistance coil of the rheostat to one of the posts of the amplifier tube that leads to the filament and the other filament post to the + or positive electrode of the A battery. This done connect the negative, that is, the zinc pole of the B battery to the positive electrode of the A battery and connect the positive, or carbon pole of the former with one end of the grid leak and connect the other end of this to the plate of the amplifier tube.
To the end of the grid leak connected with the plate of the amplifier tube connect the metal point of your crystal detector, the crystal of the latter with one post of the head phones and the other post of them with the other end of the grid leak and, finally, connect a fixed condenser in parallel with--that is across the ends of the grid leak, all of which is shown in the wiring diagram in Fig. 43.
A Grid Leak Amplifying Receiving Set With Vacuum Tube Detector.--A better amplifying receiving set can be made than the one just described by using a vacuum tube detector instead of the crystal detector. This set is built up exactly like the crystal detector described above and shown in Fig. 43 up to and including the grid leak resistance, but shunted across the latter is a vacuum tube detector, which is made and wired up precisely like the one shown at A in Fig. 41 in the chapter ahead of this one. The way a grid leak and vacuum tube detector with a one-step amplifier are connected up is shown at A in Fig. 44. Where you have a vacuum tube detector and one or more amplifying tubes connected up, or in cascade as it is called, you can use an A, or storage battery of 6 volts for all of them as shown at B in Fig. 44, but for every vacuum tube you use you must have a B or 22-1/2 volt dry battery to charge the plate with.
A Radio Frequency Transformer Amplifying Receiving Set.--Instead of using a grid leak resistance to couple up the amplifier and detector tube circuits you can use a radio frequency transformer, that is, a transformer made like a loose coupled tuning coil, and without an iron core, as shown in the wiring diagram at A in Fig. 45. In this set, which gives better results than where a grid leak is used, the amplifier tube is placed in the first oscillation circuit and the detector tube in the second circuit.
Since the radio frequency transformer has no iron core the high frequency, or radio frequency oscillating currents, as they are called, surge through it and are not changed into low frequency, or audio frequency pulsating currents, until they flow through the detector. Since the diagram shows only one amplifier and one radio frequency transformer, it is consequently a one step amplifier; however, two, three or more, amplifying tubes can be connected up by means of an equal number of radio frequency transformers when you will get wonderful results. Where a six step amplifier, that is, where six amplifying tubes are connected together, or in cascade, the first three are usually coupled up with radio frequency transformers and the last three with audio frequency transformers. A radio frequency transformer is shown at B and costs $6 to $7.
An Audio Frequency Transformer Amplifying Receiving Set.--Where audio frequency transformers are used for stepping up the voltage of the current of the detector and amplifier tubes, the radio frequency current does not get into the plate circuit of the detector at all for the reason that the iron core of the transformer chokes them off, hence, the succeeding amplifiers operate at audio frequencies. An audio frequency transformer is shown at A in Fig. 46 and a wiring diagram showing how the tubes are connected in cascade with the transformers is shown at B; it is therefore a two-step audio frequency receiving set.
A Six Step Amplifier Receiving Set With a Loop Aerial.--By using a receiving set having a three step radio frequency and a three step audio frequency, that is, a set in which there are coupled three amplifying tubes with radio frequency transformers and three amplifying tubes with audio frequency transformers as described under the caption A Radio Frequency Transformer Receiving Set, you can use a loop aerial in your room thus getting around the difficulties--if such there be--in erecting an out-door aerial. You can easily make a loop aerial by winding 10 turns of No. 14 or 16 copper wire about 1/16 inch apart on a wooden frame two feet on the side as shown in Fig. 47. With this six step amplifier set and loop aerial you can receive wave lengths of 150 to 600 meters from various high power stations which are at considerable distances away.
How to Prevent Howling.--Where radio frequency or audio frequency amplifiers are used to couple your amplifier tubes in cascade you must take particular pains to shield them from one another in order to prevent the feed back of the currents through them, which makes the head phones or loud speaker howl. To shield them from each other the tubes should be enclosed in metal boxes and placed at least 6 inches apart while the transformers should be set so that their cores are at right angles to each other and these also should be not less than six inches apart.
While a vacuum tube detector has an amplifying action of its own, and this accounts for its great sensitiveness, its amplifying action can be further increased to an enormous extent by making the radio frequency currents that are set up in the oscillation circuits react on the detector.
Such currents are called feed-back or regenerative currents and when circuits are so arranged as to cause the currents to flow back through the detector tube the amplification keeps on increasing until the capacity of the tube itself is reached. It is like using steam over and over again in a steam turbine until there is no more energy left in it. A system of circuits which will cause this regenerative action to take place is known as the Armstrong circuits and is so called after the young man who discovered it.
Since the regenerative action of the radio frequency currents is produced by the detector tube itself and which sets up an amplifying effect without the addition of an amplifying tube, this type of receiving set has found great favor with amateurs, while in combination with amplifying tubes it multiplies their power proportionately and it is in consequence used in one form or another in all the better sets.
There are many different kinds of circuits which can be used to produce the regenerative amplification effect while the various kinds of tuning coils will serve for coupling them; for instance a two or three slide single tuning coil will answer the purpose but as it does not give good results it is not advisable to spend either time or money on it. A better scheme is to use a loose coupler formed of two or three honeycomb or other compact coils, while a variocoupler or a variometer or two will produce the maximum regenerative action.
The Simplest Type of Regenerative Receiving Set. With Loose Coupled Tuning Coil.--While this regenerative set is the simplest that will give anything like fair results it is here described not on account of its desirability, but because it will serve to give you the fundamental idea of how the feed-back circuit is formed.
For this set you need: (1) a loose-coupled tuning coil such as described in Chapter III, (2) a variable condenser of .001 mfd. (microfarad) capacitance; (3) one fixed condenser of .001 mfd.; (4) one fixed condenser for the grid leak circuit of .00025 mfd.; (5) a grid leak of 1/2 to 2 megohms resistance; (6) a vacuum tube detector; (7) an A 6 volt battery; (8) a rheostat; (9) a B 22 1/2 volt battery; and (10) a pair of 2000 ohm head phones.
Connecting Up the Parts.--Begin by connecting the leading-in wire of the aerial with the binding post end of the primary coil of the loose coupler as shown in the wiring diagram Fig. 48 and then connect the sliding contact with the water pipe or other ground. Connect the binding post end of the primary coil with one post of the variable condenser, connect the other post of this with one of the posts of the .00025 mfd. condenser and the other end of this with the grid of the detector tube; then around this condenser shunt the grid leak resistance.
Next connect the sliding contact of the primary coil with the other post of the variable condenser and from this lead a wire on over to one of the terminals of the filament of the vacuum tube; to the other terminal of the filament connect one of the posts of the rheostat and connect the other post to the - or negative electrode of the A battery and then connect the + or positive electrode of it to the other terminal of the filament.
Connect the + or positive electrode of the A battery with one post of the .001 mfd. fixed condenser and connect the other post of this to one of the ends of the secondary coil of the tuning coil and which is now known as the tickler coil; then connect the other end of the secondary, or tickler coil to the plate of the vacuum tube. In the wiring diagram the secondary, or tickler coil is shown above and in a line with the primary coil but this is only for the sake of making the connections clear; in reality the secondary, or tickler coil slides to and fro in the primary coil as shown and described in Chapter III. Finally connect the negative, or zinc pole of the B battery to one side of the fixed condenser, the positive, or carbon, pole to one of the terminals of the head phones and the other terminal of this to the other post of the fixed condenser when your regenerative set is complete.
An Efficient Regenerative Receiving Set. With Three Coil Loose Coupler.--To construct a really good regenerative set you must use a loose coupled tuner that has three coils, namely a primary, a secondary and a tickler coil. A tuner of this kind is made like an ordinary loose coupled tuning coil but it has a third coil as shown at A and B in Fig. 49. The middle coil, which is the secondary, is fixed to the base, and the large outside coil, which is the primary, is movable, that is it slides to and fro over the middle coil, while the small inside coil, which is the tickler, is also movable and can slide in or out of the middle coil. None of these coils is variable; all are wound to receive waves up to 360 meters in length when used with a variable condenser of .001 mfd. capacitance. In other words you slide the coils in and out to get the right amount of coupling and you tune by adjusting the variable condenser to get the exact wave length you want.
With Compact Coils.--Compact coil tuners are formed of three fixed inductances wound in flat coils, and these are pivoted in a mounting so that the distance between them and, therefore, the coupling, can be varied, as shown at A in Fig. 50. These coils are wound up by the makers for various wave lengths ranging from a small one that will receive waves of any length up to 360 meters to a large one that has a maximum of 24,000 meters. For an amateur set get three of the smallest coils when you can not only hear amateur stations that send on a 200 meter wave but broadcasting stations that send on a 360 meter wave.
These three coils are mounted with panel plugs which latter fit into a stand, or mounting, so that the middle coil is fixed, that is, stationary, while the two outside coils can be swung to and fro like a door; this scheme permits small variations of coupling to be had between the coils and this can be done either by handles or by means of knobs on a panel board. While I have suggested the use of the smallest size coils, you can get and use those wound for any wave length you want to receive and when those are connected with variometers and variable condensers, and with a proper aerial, you will have a highly efficient receptor that will work over all ranges of wave lengths. The smallest size coils cost about $1.50 apiece and the mounting costs about $6 or $7 each.
The A Battery Potentiometer.--This device is simply a resistance like the rheostat described in connection with the preceding vacuum tube receiving sets but it is wound to 200 or 300 ohms resistance as against 1-1/2 to 6 ohms of the rheostat. It is, however, used as well as the rheostat. With a vacuum tube detector, and especially with one having a gas-content, a potentiometer is very necessary as it is only by means of it that the potential of the plate of the detector can be accurately regulated. The result of proper regulation is that when the critical potential value is reached there is a marked increase in the loudness of the sounds that are emitted by the head phones.
As you will see from A in Fig. 51 it has three taps. The two taps which are connected with the ends of the resistance coil are shunted around the A battery and the third tap, which is attached to the movable contact arm, is connected with the B battery tap, see B, at which this battery gives 18 volts. Since the A battery gives 6 volts you can vary the potential of the plate from 18 to 24 volts. The potentiometer must never be shunted around the B battery or the latter will soon run down. A potentiometer costs a couple of dollars.
The Parts and How to Connect Them Up.--For this regenerative set you will need: (1) a honeycomb or other compact three-coil tuner, (2) two variable (.001 and .0005 mfd.) condensers; (3) a .00025 mfd. fixed condenser; (4) a 1/2 to 2 megohm grid leak; (5) a tube detector; (6) a 6 volt A battery; (7) a rheostat; (8) a potentiometer; (9) an 18 or 20 volt B battery; (10) a fixed condenser of .001 mfd. fixed condenser; and (11) a pair of 2000 ohm head phones.
To wire up the parts connect the leading-in wire of the aerial with the primary coil, which is the middle one of the tuner, and connect the other terminal with the ground. Connect the ends of the secondary coil, which is the middle one, with the posts of the variable condenser and connect one of the posts of the latter with one post of the fixed .00025 mfd. condenser and the other post of this with the grid; then shunt the grid leak around it. Next connect the other post of the variable condenser to the - or negative electrode of the A battery; the + or positive electrode of this to one terminal of the detector filament and the other end of the latter to the electrode of the A battery.
Now connect one end of the tickler coil with the detector plate and the other post to the fixed .001 mfd. condenser, then the other end of this to the positive or carbon pole of the B battery.
This done shunt the potentiometer around the A battery and run a wire from the movable contact of it (the potentiometer) over to the 18 volt tap, (see B, Fig. 51), of the B battery. Finally, shunt the head phones and the .001 mfd. fixed condenser and you are ready to try out conclusions.
A Regenerative Audio Frequency Amplifier Receiving Set.--The use of amateur regenerative cascade audio frequency receiving sets is getting to be quite common. To get the greatest amplification possible with amplifying tubes you have to keep a negative potential on the grids. You can, however, get very good results without any special charging arrangement by simply connecting one post of the rheostat with the negative terminal of the filament and connecting the low potential end of the secondary of the tuning coil with the - or negative electrode of the A battery. This scheme will give the grids a negative bias of about 1 volt. You do not need to bother about these added factors that make for high efficiency until after you have got your receiving set in working order and understand all about it.
The Parts and How to Connect Them Up.--Exactly the same parts are needed for this set as the one described above, but in addition you will want: (1) two more rheostats; (2) two more sets of B 22-1/2 volt batteries; (3) two amplifier tubes, and (4) two audio frequency transformers as described in Chapter IX and pictured at A in Fig. 46.
To wire up the parts begin by connecting the leading-in wire to one end of the primary of the tuning coil and then connect the other end of the coil with the ground. A variable condenser of .001 mfd. capacitance can be connected in the ground wire, as shown in Fig. 52, to good advantage although it is not absolutely needed. Now connect one end of the secondary coil to one post of a .001 mfd. variable condenser and the other end of the secondary to the other post of the condenser.