Next bring a lead (wire) from the first post of the variable condenser over to the post of the first fixed condenser and connect the other post of the latter with the grid of the detector tube. Shunt 1/2 to 2 megohm grid leak resistance around the fixed condenser and then connect the second post of the variable condenser to one terminal of the detector tube filament. Run this wire on over and connect it with the first post of the second rheostat, the second post of which is connected with one terminal of the filament of the first amplifying tube; then connect the first post of the rheostat with one end of the secondary coil of the first audio frequency transformer, and the other end of this coil with the grid of the first amplifier tube.
Connect the lead that runs from the second post of variable condenser to the first post of the third rheostat, the second post of which is connected with one terminal of the second amplifying tube; then connect the first post of the rheostat with one end of the secondary coil of the second audio frequency transformer and the other end of this coil with the grid of the second amplifier tube.
This done connect the - or negative electrode of the A battery with the second post of the variable condenser and connect the + or positive electrode with the free post of the first rheostat, the other post of which connects with the free terminal of the filament of the detector. From this lead tap off a wire and connect it to the free terminal of the filament of the first amplifier tube, and finally connect the end of the lead with the free terminal of the filament of the second amplifier tube.
Next shunt a potentiometer around the A battery and connect the third post, which connects with the sliding contact, to the negative or zinc pole of a B battery, then connect the positive or carbon pole of it to the negative or zinc pole of a second B battery and the positive or carbon pole of the latter with one end of the primary coil of the second audio frequency transformer and the other end of it to the plate of the first amplifying tube. Run the lead on over and connect it to one of the terminals of the second fixed condenser and the other terminal of this with the plate of the second amplifying tube. Then shunt the headphones around the condenser.
Finally connect one end of the tickler coil of the tuner with the plate of the detector tube and connect the other end of the tickler to one end of the primary coil of the first audio frequency transformer and the other end of it to the wire that connects the two B batteries together.
A short wave receiving set is one that will receive a range of wave lengths of from 150 to 600 meters while the distance over which the waves can be received as well as the intensity of the sounds reproduced by the headphones depends on: (1) whether it is a regenerative set and (2) whether it is provided with amplifying tubes.
High-grade regenerative sets designed especially for receiving amateur sending stations that must use a short wave length are built on the regenerative principle just like those described in the last chapter and further amplification can be had by the use of amplifier tubes as explained in Chapter IX, but the new feature of these sets is the use of the variocoupler and one or more variometers. These tuning devices can be connected up in different ways and are very popular with amateurs at the present time.
Differing from the ordinary loose coupler the variometer has no movable contacts while the variometer is provided with taps so that you can connect it up for the wave length you want to receive. All you have to do is to tune the oscillation circuits to each other is to turn the rotor, which is the secondary coil, around in the stator, as the primary coil is called in order to get a very fine variation of the wave length. It is this construction that makes sharp tuning with these sets possible, by which is meant that all wave lengths are tuned out except the one which the receiving set is tuned for.
A Short Wave Regenerative Receiver--With One Variometer and Three Variable Condensers.--This set also includes a variocoupler and a grid coil. The way that the parts are connected together makes it a simple and at the same time a very efficient regenerative receiver for short waves. While this set can be used without shielding the parts from each other the best results are had when shields are used.
The parts you need for this set include: (1) one variocoupler; (2) one .001 microfarad variable condenser; (3) one .0005 microfarad variable condenser; (4) one .0007 microfarad variable condenser; (5) one 2 megohm grid leak; (6) one vacuum tube detector; (7) one 6 volt A battery; (8) one 6 ohm, 1-1/2 ampere rheostat; (9) one 200 ohm potentiometer; (10) one 22-1/2 volt B battery; (11) one .001 microfarad fixed condenser, (12) one pair of 2,000 ohm headphones, and (13) a variometer.
The Variocoupler.--A variocoupler consists of a primary coil wound on the outside of a tube of insulating material and to certain turns of this taps are connected so that you can fix the wave length which your aerial system is to receive from the shortest wave; i.e., 150 meters on up by steps to the longest wave, i.e., 600 meters, which is the range of most amateur variocouplers that are sold in the open market. This is the part of the variocoupler that is called the stator.
The secondary coil is wound on the section of a ball mounted on a shaft and this is swung in bearings on the stator so that it can turn in it. This part of the variocoupler is called the rotor and is arranged so that it can be mounted on a panel and adjusted by means of a knob or a dial. A diagram of a variocoupler is shown at A in Fig. 53, and the coupler itself at B. There are various makes and modifications of variocouplers on the market but all of them are about the same price which is $6.00 or $8.00.
The Variometer.--This device is quite like the variocoupler, but with these differences: (1) the rotor turns in the stator, which is also the section of a ball, and (2) one end of the primary is connected with one end of the secondary coil. To be really efficient a variometer must have a small resistance and a large inductance as well as a small dielectric loss. To secure the first two of these factors the wire should be formed of a number of fine, pure copper wires each of which is insulated and the whole strand then covered with silk. This kind of wire is the best that has yet been devised for the purpose and is sold under the trade name of litzendraht.
A new type of variometer has what is known as a basket weave, or wavy wound stator and rotor. There is no wood, insulating compound or other dielectric materials in large enough quantities to absorb the weak currents that flow between them, hence weaker sounds can be heard when this kind of a variometer is used. With it you can tune sharply to waves under 200 meters in length and up to and including wave lengths of 360 meters. When amateur stations of small power are sending on these short waves this style of variometer keeps the electric oscillations at their greatest strength and, hence, the reproduced sounds will be of maximum intensity. A wiring diagram of a variometer is shown at A in Fig. 54 and a basketball variometer is shown complete at B.
Connecting Up the Parts.--To hook-up the set connect the leading-in wire to one end of the primary coil, or stator, of the variocoupler and solder a wire to one of the taps that gives the longest wave length you want to receive. Connect the other end of this wire with one post of a .001 microfarad variable condenser and connect the other post with the ground as shown in Fig. 55. Now connect one end of the secondary coil, or rotor, to one post of a .0007 mfd. variable condenser, the other post of this to one end of the grid coil and the other end of this with the remaining end of the rotor of the variocoupler.
Next connect one post of the .0007 mfd. condenser with one of the terminals of the detector filament; then connect the other post of this condenser with one post of the .0005 mfd. variable condenser and the other post of this with the grid of the detector, then shunt the megohm grid leak around the latter condenser. This done connect the other terminal of the filament to one post of the rheostat, the other post of this to the - or negative electrode of the 6 volt A battery and the + or positive electrode of the latter to the other terminal of the filament.
Shunt the potentiometer around the A battery and connect the sliding contact with the - or zinc pole of the B battery and the + or carbon pole with one terminal of the headphone; connect the other terminal to one of the posts of the variometer and the other post of the variometer to the plate of the detector. Finally shunt a .001 mfd. fixed condenser around the headphones. If you want to amplify the current with a vacuum tube amplifier connect in the terminals of the amplifier circuit shown at A in Figs. 44 or 45 at the point where they are connected with the secondary coil of the loose coupled tuning coil, in those diagrams with the binding posts of Fig. 55 where the phones are usually connected in.
Short Wave Regenerative Receiver. With Two Variometers and Two Variable Condensers.--This type of regenerative receptor is very popular with amateurs who are using high-grade short-wave sets. When you connect up this receptor you must keep the various parts well separated. Screw the variocoupler to the middle of the base board or panel, and secure the variometers on either side of it so that the distance between them will be 9 or 10 inches. By so placing them the coupling will be the same on both sides and besides you can shield them from each other easier.
For the shield use a sheet of copper on the back of the panel and place a sheet of copper between the parts, or better, enclose the variometers and detector and amplifying tubes if you use the latter in sheet copper boxes. When you set up the variometers place them so that their stators are at right angles to each other for otherwise the magnetic lines of force set up by the coils of each one will be mutually inductive and this will make the headphones or loud speaker howl. Whatever tendency the receptor has to howl with this arrangement can be overcome by putting in a grid leak of the right resistance and adjusting the condenser.
The Parts and How to Connect Them Up.--For this set you require: (1) one variocoupler; (2) two variometers; (3) one .001 microfarad variable condenser; (4) one .0005 microfarad variable condenser; (5) one 2 megohm grid leak resistance; (6) one vacuum tube detector; (7) one 6 volt A battery; (8) one 200 ohm potentiometer; (9) one 22-1/2 volt B battery; (10) one .001 microfarad fixed condenser, and (11) one pair of 2,000 ohm headphones.
To wire up the set begin by connecting the leading-in wire to the fixed end of the primary coil, or stator, of the variocoupler, as shown in Fig. 56, and connect one post of the .001 mfd. variable condenser to the stator by soldering a short length of wire to the tap of the latter that gives the longest wave you want to receive. Now connect one end of the secondary coil, or rotor, of the variocoupler with one post of the .0005 mfd. variable condenser and the other part to the grid of the detector tube. Connect the other end of the rotor of the variocoupler to one of the posts of the first variometer and the other post of this to one of the terminals of the detector filament.
Connect this filament terminal with the - or negative electrode of the A battery and the + or positive electrode of this with one post of the rheostat and lead a wire from the other post to the free terminal of the filament. This done shunt the potential around the A battery and connect the sliding contact to the - or zinc pole of the B battery and the + or carbon pole of this to one terminal of the headphones, while the other terminal of this leads to one of the posts of the second variometer, the other post of which is connected to the plate of the detector tube. If you want to add an amplifier tube then connect it to the posts instead of the headphones as described in the foregoing set.
All receiving sets that receive over a range of wave lengths of from 150 meters to 3,000 meters are called intermediate wave sets and all sets that receive wave lengths over a range of anything more than 3,000 meters are called long wave sets. The range of intermediate wave receptors is such that they will receive amateur, broadcasting, ship and shore Navy, commercial, Arlington's time and all other stations using spark telegraph damped waves or arc or vacuum tube telephone continuous waves but not continuous wave telegraph signals, unless these have been broken up into groups at the transmitting station. To receive continuous wave telegraph signals requires receiving sets of special kind and these will be described in the next chapter.
Intermediate Wave Receiving Sets.--There are two chief schemes employed to increase the range of wave lengths that a set can receive and these are by using: (1) loading coils and shunt condensers, and (2) bank-wound coils and variable condensers. If you have a short-wave set and plan to receive intermediate waves with it then loading coils and fixed condensers shunted around them affords you the way to do it, but if you prefer to buy a new receptor then the better way is to get one with bank-wound coils and variable condensers; this latter way preserves the electrical balance of the oscillation circuits better, the electrical losses are less and the tuning easier and sharper.
Intermediate Wave Set With Loading Coils.--For this intermediate wave set you can use either of the short-wave sets described in the foregoing chapter. For the loading coils use honeycomb coils, or other good compact inductance coils, as shown in Chapter X and having a range of whatever wave length you wish to receive. The following table shows the range of wave length of the various sized coils when used with a variable condenser having a .001 microfarad capacitance, the approximate inductance of each coil in millihenries and prices at the present writing:
These and other kinds of compact coils can be bought at electrical supply houses that sell wireless goods. If your aerial is not very high or long you can use loading coils, but to get anything like efficient results with them you must have an aerial of large capacitance and the only way to get this is to put up a high and long one with two or more parallel wires spaced a goodly distance apart.
The Parts and How to Connect Them Up.--Get (1) two honeycomb or other coils of the greatest wave length you want to receive, for in order to properly balance the aerial, or primary oscillation circuit, and the closed, or secondary oscillation circuit, you have to tune them to the same wave length; (2) two .001 mfd. variable condensers, though fixed condensers will do, and (3) two small single-throw double-pole knife switches mounted on porcelain bases.
To use the loading coils all you have to do is to connect one of them in the aerial above the primary coil of the loose coupler, or variocoupler as shown in the wiring diagram in Fig. 57, then shunt one of the condensers around it and connect one of the switches around this; this switch enables you to cut in or out the loading coil at will. Likewise connect the other loading coil in one side of the closed, or secondary circuit between the variable .0007 mfd. condenser and the secondary coil of the loose coupler or variocoupler as shown in Fig. 53. The other connections are exactly the same as shown in Figs. 44 and 45.
An Intermediate Wave Set With Variocoupler Inductance Coils.--By using the coil wound on the rotor of the variocoupler as the tickler the coupling between the detector tube circuits and the aerial wire system increases as the set is tuned for greater wave lengths. This scheme makes the control of the regenerative circuit far more stable than it is where an ordinary loose coupled tuning coil is used.
When the variocoupler is adjusted for receiving very long waves the rotor sets at right angles to the stator and, since when it is in this position there is no mutual induction between them, the tickler coil serves as a loading coil for the detector plate oscillation circuit. Inductance coils for short wave lengths are usually wound in single layers but bank-wound coils, as they are called are necessary to get compactness where long wave lengths are to be received. By winding inductance coils with two or more layers the highest inductance values can be obtained with the least resistance. A wiring diagram of a multipoint inductance coil is shown in Fig. 58. You can buy this intermediate wave set assembled and ready to use or get the parts and connect them up yourself.
The Parts and How to Connect Them Up.--For this regenerative intermediate wave set get: (1) one 12 section triple bank-wound inductance coil, (2) one variometer, and (3) all the other parts shown in the diagram Fig. 58 except the variocoupler. First connect the free end of the condenser in the aerial to one of the terminals of the stator of the variocoupler; then connect the other terminal of the stator with one of the ends of the bank-wound inductance coil and connect the movable contact of this with the ground.
Next connect a wire to the aerial between the variable condenser and the stator and connect this to one post of a .0005 microfarad fixed condenser, then connect the other post of this with the grid of the detector and shunt a 2 megohm grid leak around it. Connect a wire to the ground wire between the bank-wound inductance coil and the ground proper, i.e., the radiator or water pipe, connect the other end of this to the + electrode of the A battery and connect this end also to one of the terminals of the filament. This done connect the other terminal of the filament to one post of the rheostat and the other post of this to the - or negative side of the A battery.
To the + electrode of the A battery connect the - or zinc pole of the B battery and connect the + or carbon pole of the latter with one post of the fixed .001 microfarad condenser. This done connect one terminal of the tickler coil which is on the rotor of the variometer to the plate of the detector and the other terminal of the tickler to the other post of the .001 condenser and around this shunt your headphones. Or if you want to use one or more amplifying tubes connect the circuit of the first one, see Fig. 45, to the posts on either side of the fixed condenser instead of the headphones.
A Long Wave Receiving Set.--The vivid imagination of Jules Verne never conceived anything so fascinating as the reception of messages without wires sent out by stations half way round the world; and in these days of high power cableless stations on the five continents you can listen-in to the messages and hear what is being sent out by the Lyons, Paris and other French stations, by Great Britain, Italy, Germany and even far off Russia and Japan.
A long wave set for receiving these stations must be able to tune to wave lengths up to 20,000 meters. Differing from the way in which the regenerative action of the short wave sets described in the preceding chapter is secured and which depends on a tickler coil and the coupling action of the detector in this long wave set, [Footnote: All of the short wave and intermediate wave receivers described, are connected up according to the wiring diagram used by the A. H. Grebe Company, Richmond Hill, Long Island, N. Y.] this action is obtained by the use of a tickler coil in the plate circuit which is inductively coupled to the grid circuit and this feeds back the necessary amount of current. This is a very good way to connect up the circuits for the reason that: (1) the wiring is simplified, and (2) it gives a single variable adjustment for the entire range of wave lengths the receptor is intended to cover.
The Parts and How to Connect Them Up.--The two chief features as far as the parts are concerned of this long wave length receiving set are (1) the variable condensers, and (2) the tuning inductance coils. The variable condenser used in series with the aerial wire system has 26 plates and is equal to a capacitance of .0008 mfd. which is the normal aerial capacitance. The condenser used in the secondary coil circuit has 14 plates and this is equal to a capacitance of .0004 mfd.
There are a number of inductance coils and these are arranged so that they can be connected in or cut out and combinations are thus formed which give a high efficiency and yet allow them to be compactly mounted. The inductance coils of the aerial wire system and those of the secondary coil circuit are practically alike. For wave lengths up to 2,200 meters bank litz-wound coils are used and these are wound up in 2, 4 and 6 banks in order to give the proper degree of coupling and inductance values.
Where wave lengths of more than 2,200 meters are to be received coto-coils are used as these are the "last word" in inductance coil design, and are especially adapted for medium as well as long wave lengths. [Footnote: Can be had of the Coto Coil Co., Providence, R. I.] These various coils are cut in and out by means of two five-point switches which are provided with auxiliary levers and contactors for dead-ending the right amount of the coils. In cutting in coils for increased wave lengths, that is from 10,000 to 20,000 meters, all of the coils of the aerial are connected in series as well as all of the coils of the secondary circuit. The connections for a long wave receptor are shown in the wiring diagram in Fig. 59.
Any of the receiving sets described in the foregoing chapters will respond to either: (1) a wireless telegraph transmitter that uses a spark gap and which sends out periodic electric waves, or to (2) a wireless telephone transmitter that uses an arc or a vacuum tube oscillator and which sends out continuous electric waves. To receive wireless telegraph signals, however, from a transmitter that uses an arc or a vacuum tube oscillator and which sends out continuous waves, either the transmitter or the receptor must be so constructed that the continuous waves will be broken up into groups of audio frequency and this is done in several different ways.
There are four different ways employed at the present time to break up the continuous waves of a wireless telegraph transmitter into groups and these are: (a) the heterodyne, or beat, method, in which waves of different lengths are impressed on the received waves and so produces beats; (b) the tikker, or chopper method, in which the high frequency currents are rapidly broken up; (c) the variable condenser method, in which the movable plates are made to rapidly rotate; (d) the tone wheel, or frequency transformer, as it is often called, and which is really a modified form of and an improvement on the tikker. The heterodyne method will be described in this chapter.
What the Heterodyne or Beat Method Is.--The word heterodyne was coined from the Greek words heteros which means other, or different, and dyne which means power; in other words it means when used in connection with a wireless receptor that another and different high frequency current is used besides the one that is received from the sending station. In music a beat means a regularly recurrent swelling caused by the reinforcement of a sound and this is set up by the interference of sound waves which have slightly different periods of vibration as, for instance, when two tones take place that are not quite in tune with each other. This, then, is the principle of the heterodyne, or beat, receptor.
In the heterodyne, or beat method, separate sustained oscillations, that are just about as strong as those of the incoming waves, are set up in the receiving circuits and their frequency is just a little higher or a little lower than those that are set up by the waves received from the distant transmitter. The result is that these oscillations of different frequencies interfere and reinforce each other when beats are produced, the period of which is slow enough to be heard in the headphones, hence the incoming signals can be heard only when waves from the sending station are being received. A fuller explanation of how this is done will be found in Chapter XV.
The Autodyne or Self-Heterodyne Long-Wave Receiving Set.--This is the simplest type of heterodyne receptor and it will receive periodic waves from spark telegraph transmitters or continuous waves from an arc or vacuum tube telegraph transmitter. In this type of receptor the detector tube itself is made to set up the heterodyne oscillations which interfere with those that are produced by the incoming waves that are a little out of tune with it.
With a long wave autodyne, or self-heterodyne receptor, as this type is called, and a two-step audio-frequency amplifier you can clearly hear many of the cableless stations of Europe and others that send out long waves. For receiving long wave stations, however, you must have a long aerial--a single wire 200 or more feet in length will do--and the higher it is the louder will be the signals. Where it is not possible to put the aerial up a hundred feet or more above the ground, you can use a lower one and still get messages in International Morse fairly strong.
The Parts and Connections of an Autodyne, or Self-Heterodyne, Receiving Set.--For this long wave receiving set you will need: (1) one variocoupler with the primary coil wound on the stator and the secondary coil and tickler coil wound on the rotor, or you can use three honeycomb or other good compact coils of the longest wave you want to receive, a table of which is given in Chapter XII; (2) two .001 mfd. variable condensers; (3) one .0005 mfd. variable condenser; (4) one .5 to 2 megohm grid leak resistance; (5) one vacuum tube detector; (6) one A battery; (7) one rheostat; (8) one B battery; (9) one potentiometer; (10) one .001 mfd. fixed condenser and (11) one pair of headphones. For the two-step amplifier you must, of course, have besides the above parts the amplifier tubes, variable condensers, batteries rheostats, potentiometers and fixed condensers as explained in Chapter IX. The connections for the autodyne, or self-heterodyne, receiving set are shown in Fig. 60.
The Separate Heterodyne Long Wave Receiving Set.--This is a better long wave receptor than the self heterodyne set described above for receiving wireless telegraph signals sent out by a continuous long wave transmitter. The great advantage of using a separate vacuum tube to generate the heterodyne oscillations is that you can make the frequency of the oscillations just what you want it to be and hence you can make it a little higher or a little lower than the oscillations set up by the received waves.
The Parts and Connections of a Separate Heterodyne Long Wave Receiving Set.--The parts required for this long wave receiving set are: (1) four honeycomb or other good compact inductance coils of the longest wave length that you want to receive; (2) three .001 mfd. variable condensers; (3) one .0005 mfd. variable condenser; (4) one 1 megohm grid leak resistance; (5) one vacuum tube detector; (6) one A battery; (7) two rheostats; (8) two B batteries, one of which is supplied with taps; (9) one potentiometer; (10) one vacuum tube amplifier, for setting up the heterodyne oscillations; (11) a pair of headphones and (12) all of the parts for a two-step amplifier as detailed in Chapter IX, that is if you are going to use amplifiers. The connections are shown in Fig. 61.
In using either of these heterodyne receivers be sure to carefully adjust the B battery by means of the potentiometer.
[Footnote: The amplifier tube in this case is used as a generator of oscillations.]
Wireless Headphones.--A telephone receiver for a wireless receiving set is made exactly on the same principle as an ordinary Bell telephone receiver. The only difference between them is that the former is made flat and compact so that a pair of them can be fastened together with a band and worn on the head (when it is called a headset), while the latter is long and cylindrical so that it can be held to the ear. A further difference between them is that the wireless headphone is made as sensitive as possible so that it will respond to very feeble currents, while the ordinary telephone receiver is far from being sensitive and will respond only to comparatively large currents.
How a Bell Telephone Receiver Is Made.--An ordinary telephone receiver consists of three chief parts and these are: (1) a hard-rubber, or composition, shell and cap, (2) a permanent steel bar magnet on one end of which is wound a coil of fine insulated copper wire, and (3) a soft iron disk, or diaphragm, all of which are shown in the cross-section in Fig. 62. The bar magnet is securely fixed inside of the handle so that the outside end comes to within about 1/32 of an inch of the diaphragm when this is laid on top of the shell and the cap is screwed on.
| Photograph unavailable |
| original © Underwood and Underwood. Alexander Graham Bell, Inventor of the Telephone, now an ardent Radio Enthusiast. |
The ends of the coil of wire are connected with two binding posts which are in the end of the shell, but are shown in the picture at the sides for the sake of clearness. This coil usually has a resistance of about 75 ohms and the meaning of the ohmic resistance of a receiver and its bearing on the sensitiveness of it will be explained a little farther along. After the disk, or diaphragm, which is generally made of thin, soft sheet iron that has been tinned or japanned, [Footnote: A disk of photographic tin-type plate is generally used.] is placed over the end of the magnet, the cap, which has a small opening in it, is screwed on and the receiver is ready to use.
How a Wireless Headphone Is Made.--For wireless work a receiver of the watch-case type is used and nearly always two such receivers are connected with a headband. It consists of a permanent bar magnet bent so that it will fit into the shell of the receiver as shown at A in Fig. 63.
The ends of this magnet, which are called poles, are bent up, and hence this type is called a bipolar receiver. The magnets are wound with fine insulated wire as before and the diaphragm is held securely in place over them by screwing on the cap.
About Resistance, Turns of Wire and Sensitivity of Headphones.--If you are a beginner in wireless you will hear those who are experienced speak of a telephone receiver as having a resistance of 75 ohms, 1,000 ohms, 2,000 or 3,000 ohms, as the case may be; from this you will gather that the higher the resistance of the wire on the magnets the more sensitive the receiver is. In a sense this is true, but it is not the resistance of the magnet coils that makes it sensitive, in fact, it cuts down the current, but it is the number of turns of wire on them that determines its sensitiveness; it is easy to see that this is so, for the larger the number of turns the more often will the same current flow round the cores of the magnet and so magnetize them to a greater extent.
But to wind a large number of turns of wire close enough to the cores to be effective the wire must be very small and so, of course, the higher the resistance will be. Now the wire used for winding good receivers is usually No. 40, and this has a diameter of .0031 inch; consequently, when you know the ohmic resistance you get an idea of the number of turns of wire and from this you gather in a general way what the sensitivity of the receiver is.
A receiver that is sensitive enough for wireless work should be wound to not less than 1,000 ohms (this means each ear phone), while those of a better grade are wound to as high as 3,000 ohms for each one. A high-grade headset is shown in Fig. 64. Each phone of a headset should be wound to the same resistance, and these are connected in series as shown. Where two or more headsets are used with one wireless receiving set they must all be of the same resistance and connected in series, that is, the coils of one head set are connected with the coils of the next head set and so on to form a continuous circuit.
The Impedance of Headphones.--When a current is flowing through a circuit the material of which the wire is made not only opposes its passage--this is called its ohmic resistance--but a counter-electromotive force to the current is set up due to the inductive effects of the current on itself and this is called impedance. Where a wire is wound in a coil the impedance of the circuit is increased and where an alternating current is used the impedance grows greater as the frequency gets higher. The impedance of the magnet coils of a receiver is so great for high frequency oscillations that the latter cannot pass through them; in other words, they are choked off.
How the Headphones Work.--As you will see from the cross-sections in Figs. 62 and 63 there is no connection, electrical or mechanical, between the diaphragm and the other parts of the receiver. Now when either feeble oscillations, which have been rectified by a detector, or small currents from a B battery, flow through the magnet coils the permanent steel magnet is energized to a greater extent than when no current is flowing through it. This added magnetic energy makes the magnet attract the diaphragm more than it would do by its own force. If, on the other hand, the current is cut off the pull of the magnet is lessened and as its attraction for the diaphragm is decreased the latter springs back to its original position. When varying currents flow through the coils the diaphragm vibrates accordingly and sends out sound waves.
About Loud Speakers.--The simplest acoustic instrument ever invented is the megaphone, which latter is a Greek word meaning great sound. It is a very primitive device and our Indians made it out of birch-bark before Columbus discovered America. In its simplest form it consists of a cone-shaped horn and as the speaker talks into the small end the concentrated sound waves pass out of the large end in whatever direction it is held.
Now a loud speaker of whatever kind consists of two chief parts and these are: (1) a telephone receiver, and (2) a megaphone, or horn as it is called. A loud speaker when connected with a wireless receiving set makes it possible for a room, or an auditorium, full of people, or an outdoor crowd, to hear what is being sent out by a distant station instead of being limited to a few persons listening-in with headphones. To use a loud speaker you should have a vacuum tube detector receiving set and this must be provided with a one-step amplifier at least.
To get really good results you need a two-step amplifier and then energize the plate of the second vacuum tube amplifier with a 100 volt B battery; or if you have a three-step amplifier then use the high voltage on the plate of the third amplifier tube. Amplifying tubes are made to stand a plate potential of 100 volts and this is the kind you must use. Now it may seem curious, but when the current flows through the coils of the telephone receiver in one direction it gives better results than when it flows through in the other direction; to find out the way the current gives the best results try it out both ways and this you can do by simply reversing the connections.
The Simplest Type of Loud Speaker.--This loud speaker, which is called, the Arkay, [Footnote: Made by the Riley-Klotz Mfg. Co., Newark, N. J.] will work on a one- or two-step amplifier. It consists of a brass horn with a curve in it and in the bottom there is an adapter, or frame, with a set screw in it so that you can fit in one of your headphones and this is all there is to it. The construction is rigid enough to prevent overtones, or distortion of speech or music. It is shown in Fig. 65.
Another Simple Kind of Loud Speaker.--Another loud speaker, see Fig. 66, is known as the Amplitone [Footnote: Made by the American Pattern, Foundry and Machine Co., 82 Church Street, N. Y. C.] and it likewise makes use of the headphones as the sound producer. This device has a cast metal horn which improves the quality of the sound, and all you have to do is to slip the headphones on the inlet tubes of the horn and it is ready for use. The two headphones not only give a longer volume of sound than where a single one is used but there is a certain blended quality which results from one phone smoothing out the imperfections of the other.
A Third Kind of Simple Loud Speaker.--The operation of the Amplitron, [Footnote: Made by the Radio Service Co., 110 W. 40th Street, N. Y.] as this loud speaker is called, is slightly different from others used for the same purpose. The sounds set up by the headphone are conveyed to the apex of an inverted copper cone which is 7 inches long and 10 inches in diameter. Here it is reflected by a parabolic mirror which greatly amplifies the sounds. The amplification takes place without distortion, the sounds remaining as clear and crisp as when projected by the transmitting station. By removing the cap from the receiver the shell is screwed into a receptacle on the end of the loud speaker and the instrument is ready for use. It is pictured in Fig. 67.
A Super Loud Speaker.--This loud speaker, which is known as the Magnavox Telemegafone, was the instrument used by Lt. Herbert E. Metcalf, 3,000 feet in the air, and which startled the City of Washington on April 2, 1919, by repeating President Wilson's Victory Loan Message from an airplane in flight so that it was distinctly heard by 20,000 people below.
This wonderful achievement was accomplished through the installation of the Magnavox and amplifiers in front of the Treasury Building. Every word Lt. Metcalf spoke into his wireless telephone transmitter was caught and swelled in volume by the Telemegafones below and persons blocks away could hear the message plainly. Two kinds of these loud speakers are made and these are: (1) a small loud speaker for the use of operators so that headphones need not be worn, and (2) a large loud speaker for auditorium and out-door audiences.
| Photograph unavailable |
| original © Underwood and Underwood. World's Largest Loud Speaker ever made. Installed in Lytle Park, Cincinnati, Ohio, to permit President Harding's Address at Point Pleasant, Ohio, during the Grant Centenary Celebration to be heard within a radius of one square. |
Either kind may be used with a one- or two-step amplifier or with a cascade of half a dozen amplifiers, according to the degree of loudness desired. The Telemegafone itself is not an amplifier in the true sense inasmuch as it contains no elements which will locally increase the incoming current. It does, however, transform the variable electric currents of the wireless receiving set into sound vibrations in a most wonderful manner.
A telemegafone of either kind is formed of: (1) a telephone receiver of large proportions, (2) a step-down induction coil, and (3) a 6 volt storage battery that energizes a powerful electromagnet which works the diaphragm. An electromagnet is used instead of a permanent magnet and this is energized by a 6-volt storage battery as shown in the wiring diagram at A in Fig. 68. One end of the core of this magnet is fixed to the iron case of the speaker and together these form the equivalent of a horseshoe magnet. A movable coil of wire is supported from the center of the diaphragm the edge of which is rigidly held between the case and the small end of the horn. This coil is placed over the upper end of the magnet and its terminals are connected to the secondary of the induction coil. Now when the coil is energized by the current from the amplifiers it and the core act like a solenoid in that the coil tends to suck the core into it; but since the core is fixed and the coil is movable the core draws the coil down instead. The result is that with every variation of the current that flows through the coil it moves up and down and pulls and pushes the diaphragm down and up with it. The large amplitude of the vibrations of the latter set up powerful sound waves which can be heard several blocks away from the horn. In this way then are the faint incoming signals, speech and music which are received by the amplifying receiving set reproduced and magnified enormously. The Telemegafone is shown complete at B.